Quarterly Journal of Microscopical Science, Vol s2-73, 289-331, Copyright © 1929 by Company of Biologists

Memoirs: On the Development of the Skull of Leptodeira Hotamboia

The cartilaginous cranium of the snake, Leptodeira hotamboia, consists of basal plate and trabeculae, otic, and nasal capsules.

The crista sellaris of the basal plate is situated exceptionally far in front of the otic capsules, and the basicranial fenestra lies entirely in an anterior pro-otic third of the basal plate.

The trabeculae converge but do not fuse in front of the fenestra hypophyseos. They run forward parallel to each other, and fuse to form the nasal septum in the nasal region.

The otic capsules show a large vestibular division and a smaller cochlear portion. Posteriorly each capsule is separated from the basal plate by the fissura metotica. This fissure is divided into a small anterior medial opening of the recessus scalae tympani and a posterior jugular foramen. The vagus nerve passes through the posterior division, but the jugular vein, as in reptiles generally, passes out through the foramen magnum. The fenestra cochleae, an aperture of the cochlear capsule, faces towards the recessus scalae tympani.

The fenestra cochleae of the reptile is compared with the fenestra rotunda of the mammal, and it is found that the two are not homologous. The apertura lateralis of the recessus scalae tympani of the reptile is the homologue of the fenestra rotunda of the mammal. The secondary tympanic membranes in the two classes correspond morphologically as well as physiologically.

This conclusion does not confirm Gaupp's hypothesis (1900), but is in keeping with the earlier suggestion of Versluys (1898). The columella auris consists of foot-plate and shaft. The distal end of the shaft is bent sharply backwards, is elongated, and is in contact with the quadrate. A small nodule at the distal end ossifies separately from the columella as a process of the quadrate. This is probably an intercalare, and the distal bent end of the columella would then represent the dorsal process of the lizard.

There is no interorbital septum. The cartilaginous side-walls in the orbito-temporal region are lacking, and are compensated for by strong downgrowths of the parietals and frontals. The eye-muscles and their nerves are gathered together in the space behind these bones and pass out through a common opening, the foramen orbitale magnum.

A small basitrabecular process projects laterally from the trabecular plate. It supports a small bone situated in the sidewall of the skull over the trigeminal incisure and the facial foramen. The bone is not performed in cartilage, but its relations to the nerves and blood-vessels show it to be an epipterygoid. This basitrabecular process and epipterygoid evidently correspond to the so-called ‘alisphenoid’ of Parker (1878) and Peyer (1912).

The nasal capsules are delicate and incomplete. There are large conchal infoldings. The cartilaginous cup of Jacobson's organ is isolated from the rest of the nasal skeleton. A small planum antorbitale is present, and is attached to the dorsal edge of the nasal septum by a posterior commissure.

Membrane bones are strongly developed. Parietals, frontals, and parasphenoid form a strong bony case which gives a firm foundation for the working of the slender and loosely articulated palatal and jaw structures.