1891.] HELODERMA HORRIDUM AND H. SUSPECTUM. Ill 



same size, looks very diagrammatic ; the outer prsemaxiUary teeth 

 are always larger than 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 me as well as in those described 

 by Troschel and by Kaup, who first noticed tiieir 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. My adult sppcimen 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 U. horridum a 

 small azygous ossification in the cartilage of the mandibular sym- 

 physis, apparently the homologue of the symphysial (mento-mecke- 

 lian) bones of most tailless Batrachians. On referring to DoUo's 

 paper on the skull oi Iguanodon ', I find that an apparently similar 

 ossification lias been noticed by KollikerMn Man. Whether the 

 prepsymphysial 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 ^ The con- 

 Fig. 2. 



Symphysial portion of mandible of H. horridnm. 

 Lower Tiew ; 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 KoUiker, 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 li. horridum 

 is the presence of a small tooth-like bony knob on the anterior por- 

 tion of the under surface of the basioccipital, such as is often found, 



1 Bull. Mus. Belg. ii. 1883, p. 224. 



- Entwickelungsgeschichte des Menscnen, &c., 2nd ed. p. 473. 



^ Am. Journ. Sci. xx.xix. 1890, pp. 81, 418. 



