If now we take up a consideration of each of the characters mentioned by Lydekker we find that the first one holds good for all examples of the group. The second character, "without limbs," is not good. Species of Œstocephalus, Ptyonius, Molgophis all possess limbs; and doubtless Ophiderpeton will be found to possess limbs also, since it has a well-developed pectoral girdle. The limbs in all of these genera are small. The third character, "apparently without pectoral and pelvic girdles," is not at all a good character, since nearly every specimen of some species and almost all species show evidences of pectoral girdles and a few exhibit pelvic girdles. The fourth character, "vertebræ with elongated centra and aborted neural spines," is not a good distinguishing character, since Amphibamus, an undoubted microsaurian, possesses the same vertebral characters. "Ribs slender, and barbed like those of fishes" is a character which is common to several widely distinct genera. All Microsauria possess long, slender ribs, and the barbed condition is one which is possessed by only a few, Thyrsidium, Ophiderpeton, etc., the so-called "barb" being merely a highly exaggerated tuberculum. The teeth of nearly all Microsauria are smooth, so that the character "teeth smooth" is not a good one for a group definition. It has not been possible to examine any of the American specimens for the plication of the dentine, since the forms are so rare and the fossils very fragile. The last character, "external gills probably persistent," is certainly not true for the American species, and the evidence for the European species is negative. Fritsch described and figured (Fauna der Gaskohle, Bd. I, 1883, p. 114, Tafeln 18 and 23) structures which he regarded as supporting structures for the external branchiæ. He says in regard to these structures:
"Bei dem Umstande, dass sie von der Kiemengegend aus sich büschelförmig verbreiten und man ihren Contakt mit einer Art von Branchiæ constatiren kann, zweifle ich nicht daran, dass diese Stäbchen dem Kiemenapparate angehören. Bedenklich ist nur ihre grosse Zahl und das Vorkommen bis zum 16ten Wirbel und ich erwog die Möglichkeit, dass diese Stäbchen einem zarten Bauchpanzer angehören könnten. Da aber weiter im Verlaufe der ganzen Wirbelsäule nichts Aehnliches vorkömmt, so ist man gezwungen anzunehmen, dass Dolichosoma sehr grosse lange Kiemenbüschel besessen haben muss."
John Samuel Budgett ([79, p. 162]), in his discussion of the "Structure of the Larval Polypterus," refers to the above-described specimen of "aistopodous Stegocephali," i.e., Dolichosoma longissimum Fritsch, and calls especial attention to the similarity of the external rod of segmented cartilage on the hyomandibular of Polypterus to this structure, to which Fritsch has assigned a branchiate nature in Dolichosoma. There is no doubt in the mind of the present writer, however, that the rod of cartilage, referred by Fritsch to the gills, can be other than scutellate rods of the ventral armature, these rods belonging to the armature of the breast or throat. The evidence for this conclusion is furnished by Fritsch himself (Fauna der Gaskohle, Bd. I, plate 18, fig. 11), where all may read in the figure of the specimen the facts of the case. There is quite evidently no justification for Fritsch's conclusion of the branchiate nature of Dolichosoma. There is no evidence of any gill-like structure in the American snake-like amphibians of the Coal Measures.
Reviewing, then, the characters of the group which have been assigned by various observers, it will be seen that there is but a single character which holds good: "body long and snake-like." This is totally insufficient for the retention of the group. I therefore propose to abolish the group entirely from zoological classification. It is not even a family. It will, however, be convenient to refer to the snake-like forms as "aistopodous."
[CHAPTER XIII.]
THE MICROSAURIAN FAMILY HYLONOMIDÆ, FROM THE COAL MEASURES OF NOVA SCOTIA.
Family HYLONOMIDÆ Fritsch, 1883.
Fritsch, Fauna der Gaskohle und der Kalksteine der Permformations Böhmens, Bd. 1, p. 159, 1883.