Snakes, Ophidia—regarded by some authorities as an order of the class Reptilia, by the author as a sub-order of the order Squamata, which includes besides the Lizards, Lacertilia, the Chameleons, Rhiptoglossa, and the extinct Dolichosauria and Mosasauria—may be defined as greatly elongate scaly Reptiles without limbs, or with mere vestiges of the hind pair, without movable eyelids, without ear-opening, with elongate, deeply forked tongue retractile into a basal sheath, with transverse vent and paired copulatory organs, and with the two halves of the lower jaw independently movable, connected at the symphysis by an elastic ligament.

The latter character alone distinguishes them from all Lizards, but no single Lizard possesses all the others in combination.

In their most highly developed form these Reptiles are adapted for rapid reptation and for swallowing prey much exceeding their own calibre; hence the bones of the skull, on which a prehensile function devolves, are loosely attached to the cranium by ligamentous elastic tissue, or articulated in such a manner as to permit a wide buccal expansion; whilst the absence of a sternum and the mobile attachment of the ribs allow a corresponding dilatation of the body as the prey descends into the digestive canal.

The fatal venom which many of these Reptiles possess has so impressed the mind of men, even the scientific, that for a long time snakes were primarily divided into poisonous and non-poisonous, a classification in which the more important characters, derived from the general structure, and especially from the skull, were subordinated to the physiological. Such a system was far from reflecting natural relationships. Besides, as our knowledge progressed, drawing a distinction between poisonous and harmless snakes became more and more difficult, so many snakes previously regarded as harmless proving to be poisonous in various degrees—at least enough to paralyze the small prey on which they subsist, if not to be of serious danger to man.

In the division into families, as followed in this work, the presence or absence of a poison organ is left out of consideration. Further, in this as in many other groups of the animal kingdom, external characters do not furnish trustworthy indications for higher divisions, and the definitions of the families are therefore based exclusively on osteological characters. For those who wish to name snakes with facility, the key which concludes the chapter on External Characters will, however, remedy this defect, and suffice for the identification of all the European species without any reference to their anatomy. Many attempts have been made to furnish an easy criterion for the distinction of harmless from poisonous snakes, but the characters hitherto suggested with this object can only be applied successfully to the small number of representatives in a limited area. Thus, in Southern Australia it might be stated that all snakes showing the regular nine large shields on the upper surface of the head are dangerous to man, whilst those with small shields or scales are harmless; but in most parts of Europe this criterion would have to be reversed. In some countries the shape of the pupil might be used for the purpose, in others the size of the ventral shields, or the presence or absence of a loreal shield, between the nasal and the preocular, and so on. But when we have to deal with the snakes of the whole world, about 2,000 species, of which nearly one-third are poisonous to a greater or less degree, every attempt at a definition of the two categories without regard to the dentition breaks down. Only those who have made a study of the snakes of the world can make a guess from the general appearance as to an unknown form being poisonous or not, and even they may sometimes feel embarrassed, unless the dentition be examined; the mistakes which have occasionally been made by some experienced herpetologists are proof sufficient of the fallacy of external characters for this purpose.

The Ophidia are divided into nine families, the first, third, seventh, and ninth of which have representatives in Europe:

I. No transverse (ectopterygoid) bone; pterygoid not extending to quadrate or mandible; no supratemporal; nasals in contact with prefrontals; coronoid present; vestiges of pelvis.

Maxillary loosely attached to lower surface of cranium, toothed; lower jaw edentulous; a single pelvic bone

Maxillary bordering mouth, forming a suture with premaxillary, prefrontal, and frontal, toothless; pubis and ischium present, latter forming a symphysis

II. Transverse bone present; both jaws toothed.

A. Coronoid present; nasals in contact with prefrontals.

1. Vestiges of pelvis; supratemporal present.

Supratemporal large, suspending quadrate

(Subfamilies: PythoninÆ, BoinÆ.)

Supratemporal small, intercalated in the cranial wall

2. No vestiges of pelvis; supratemporal absent

B. Coronoid absent; supratemporal present.

1. Maxillary horizontal; pterygoids reaching quadrate or mandible.

Nasals in contact with prefrontals

Nasals not in contact with prefrontals

Three series: A. Aglypha (subfamilies: AcrochordinÆ, ColubrinÆ, DasypeltinÆ); B. Opisthoglypha (HomalopsinÆ, DipsadomorphinÆ, ElachistodontinÆ); C. Proteroglypha (HydrophiinÆ, ElapinÆ).

2. Maxillary horizontal, converging posteriorly towards palatine; pterygoid not reaching quadrate or mandible

3. Maxillary vertically erectile perpendicularly to transverse bone; pterygoid reaching quadrate or mandible

(Subfamilies: ViperinÆ, CrotalinÆ.)

The technical terms employed in the above synopsis will be found explained and illustrated by figures in the chapter on the Skeleton.

No serial arrangement can express the affinities of the various groups as conceived by the classificator; a diagram therefore follows to show the author’s views as to their interrelationships, and possibly their phylogeny. Leaving aside the TyphlopidÆ and GlauconiidÆ, which should be regarded as burrowing types independently derived from some Ophidian form less specialized than any with which we are at present acquainted, and probably without direct relationship to the Lizards, the family BoidÆ, and more especially the Pythons, claim the position of ancestral group, from which all other snakes may have been derived.

Further remarks on this subject in the chapter on Dentition.

It is to be regretted that paleontology cannot help us at present as concerns the lines of evolution, the comparatively few fossil Ophidians known, from the Lower Eocene upwards, the remains of which can be identified with some measure of certainty, being either non-poisonous types (BoidÆ, IlysiidÆ, PalÆophiidÆ, ColubridÆ) or ViperidÆ (Viperines from the Miocene of France and Germany, Crotalines from the Miocene of North America). The vertebrÆ from the Puerco Eocene of America, on the limit between the Cretaceous and Eocene periods, described as the oldest snake remains, Helagras, Cope, are stated to approach the Lacertilian type.

Whether the vertebrÆ named Symoliophis, Sauvage, from the chalk of France, and Coniophis, Marsh, from the Laramie Cretaceous of North America, are Ophidian, as claimed by their describers, or Dolichosaurian, cannot be decided without further material.