OCR Text |
Show 1891.] HELODERMA HORRIDUM AND H. SUSPECTUM. Ill same size, looks very diagrammatic ; the outer prsemaxillary teeth are always larger tban the median. The postorbital arch is more slender in H. suspectum. Palatine and pterygoid teeth appear to be constantly absent in H. suspectum, whereas they are present in both skulls of H. horridum examined by m e as well as in those described by Troschel and by Kaup, who first noticed their presence. It is remarkable to find so important a difference between two species so closely allied. The presence of palatine teeth is quite exceptional among Lacertilia; they are only known in two other genera, viz., Ophisaurvs and Chamceleolis. M y adult specimen has six or seven teeth on each pterygoid and three on each palatine; the younger specimen has only one palatine and two pterygoid teeth on each side. I was much surprised to find on the adult skull of H. horridum a small azygous ossification in the cartilage of the mandibular symphysis, apparently the homologue of the symphysial (mento-mecke- Iian) bones of most tailless Batrachians. On referring to Dollo's paper on the skull of Lguanodon \ I find that an apparently similar ossification has been noticed by Kolliker2 in Man. Whether the pra^symphysial bone (Dollo) of Dinosaurs is homologous with this is still questionable, especially since the discovery of a corresponding bone in the upper jaw, the " rostral bone " of Marsh 3. The con- Symphysial portion of mandible of H. horridum. Lower view ; nat. size. dition of this little bone in Heloderma appears to be very much the same as in Man, as far as may be judged from the few words devoted to it by Kolliker, and it is likewise only of exceptional occurrence, as I have failed to find a trace of it in any but this solitary specimen. Another individual peculiarity of this same skull of H. horridum is the presence of a small tooth-like bony knob on the anterior portion of the under surface of the basioccipital, such as is often found, ' Bull. Mus. Belg. ii. 1883, p. 224. 2 Entwickelungsgeschichte des Menschen, &c, 2nd ed. p. 473. 3 Am. Journ. Sci. xxxix. 1890, pp. 81, 418. |