214 DR. R. W. SHUFELDT ON [-^-pr. 1, 



passes out of the pelvis and down the pelvic limh, 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 si.nple mode of sacral nerve- 

 interlacement. Gegenbaur, in liis ' Elements of Comparative Ana- 

 tomy ' (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 hefore us. On the other hand, according 

 to Hoffmann (45), the sacral interlacement in such forms as Alligator 

 mississipiensis, Cyclodus boddaerti, Hydrosaurus marmoratus, and 

 Monitor indicus is conspicuously intricate, the more especially in such 

 a form as the Alligator (see Taf. Ixxxvii. in tbe wurk 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 interlace- 

 ment of the spiiial nerve-plexuses is complicated ; and this vrould seem 

 to point to the fact that in the case of Heloderma, wherein the inter- 

 lacement of those plexuses is most simple, it is most probably affined 

 with a far more lowly order of Reptiles, 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 of Heloderma suspectum 1 

 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 vertebrae are 

 prot'oelous, 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, fi;ially, 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 



' [68] 



