OCR Text |
Show 214 DR. R. W. SHUFELDT ON [Apr. 1, passes out of the pelvis and down the pelvic limb, dividing up into branches to supply the muscles of the extremity. Either of the two posterior trunks of the sacral plexus distribute one or more nerve-branches to the pelvo-crural group of muscles, these branches being thrown off both prior and subsequent to their mergence with each other. Now I am not familiar with any Lizard wherewith to compare Heloderma in the matter of its very simple mode of sacral nerve-interlacement. Gegenbaur, in his ' JElements of Comparative Anatomy ' (English edition, p. 434), presents us with a diagram (fig. 227) intending to indicate the most usual arrangement of the sacral plexus in a reptile, and, although it is quite simple, it is not so simple as it is in the subject we have before us. On the other hand, according to Hoffmann (45), the sacral interlacement in such forms as Alligator mississipiensis, Cyclodus boddaerti, Ilydrosaurus marmoratus, and Monitor indicus is conspicuously intricate, the more especially in such a form as the Alligator (see Taf. lxxxvii. in the work quoted). To a certain extent this must have its significance, as in the Crocodilia we recognize a group of Reptiles that structurally stand the highest of the class to which they belong, and in them the mode of interlacement of the spinal nerve-plexuses is complicated ; and this would seem to point to the fact that in the case of Heloderma, wherein the interlacement of those plexuses is most simple, it is most probably affined with a far more lowly order of Beptiles, perhaps with some of the very lowest of existing North-American types. XIII. OF THE SKELETON. The Vertebral Column.-Upon counting the vertebrae composing the spinal column of an adult specimen oi Heloderma suspectum I found that there were in all sixty-four of them. Of these eight belonged to the cervical division of the column, twenty-two to the dorsal, five to the lumbar, two sacral, and twenty-seven in the tail or caudal division. In character these vertebras are procoelons, the more spherical cups and balls being seen in mid-cervical region, while those of the transversely elliptical pattern are best developed in the dorsal portion of the column ; and, finally, the more rudimentary ones are devoted to the ultimate joints as we gradually pass to the end of the tail. Commencing with the atlas it is found to be composed of five separate pieces; three of these are devoted to the formation of its anterior cup for the cranial condyle. Of these three pieces, one is a mid-ventral one, while either of the others are ventro-laterally situated. Each side of the neural arch is formed by one of the two of the remaining pieces of the five of the component elements of this vertebra; and in a large specimen of this lizard none of these five parts had co-ossified. A proatlas does not seem to exist in Heloderma. Turning to the axis vertebra we find it characterized by a very long and prominent neural spine ; indeed, its length distinguishes it from any other vertebra in the column. Its odontoid process is conical |