1066 REPORT OF COMMISSIONER OF FISH AND FISHERIES. [86] 



These higher groups seem to fall into six arbitrary categories, habitual 

 movement in particular ways having brought about the special modifi- 

 cations. 



Professor Marsh* in speculating upon the origin of the forms of the 

 vertebrte of the different groui)s of Vertebrates reaches the following 

 conclusions: 



" (1 ) Biconcave vertehrce (Fishes and Amphibians) : the primitive type; 

 a weak articulation, admitting free but limited motion. From this form 

 liave been directly derived the other varieties, namely : 



" (2) Plane vertehrce (Mammals) : affording a stronger joint, with mo- 

 tion still restricted. 



" (3) Gup and hall vertehrce (Reptiles) : a strong and flexible joint, well 

 fitted for general motion, and evidently produced by it. The vertebrae 

 are procoilian when lateral motion is dominant (serpents); opisthocoelian 

 with varied motion (Dinosaur cervicals). 



" (4) Saddle vertehrce (Birds) : the highest type ; a very strong and free 

 articulation, especially adapted to motion in a vertical plane, and mainly 

 <lue originally to its predominance. (Is predominant in the cervical 

 series.)" 



Professor Cope has found a fifth type of vertebral column in a rep- 

 tilian type from the Permian formation of Texas, in which there are lat- 

 eral and inferior intercentral pieces wedged in between the true centra 

 of sucessive vertebrae. The vertebral column of this type, which rep- 

 resents a very important group, which he calls Theromorpha, thought 

 by him to be ancestrally related to the oviparous Monotremes, he thinks 

 was developed by the peculiar lateral flexures to which the vertebral 

 axis was subjected. Further studies may throw some additional light 

 upon the mode in which the supplementary intercentra were formed in 

 this group, as special or peculiar conditions have evidently determined 

 this singular raorjjhological differentiation. 



A sixth type is found in armor-bearing forms, as in the extinct Arma- 

 dillos and the recent and fossil emydoid Testudinata, which has been 

 developed in consequence of a loss of mobility of the axial column due 

 to the existence of an inflexible outer carapace, as pointed out by Spen- 

 cer and myself.t In what way this degeneration of the centra has been 

 developed in the fossil Armadillos has been discussed by me in the 

 2)aper cited, as follows: "The carapace was supported for nearly half 

 its length upon the haunch bones (ilia and ischia), as well as by the 

 strong, longitudinal, median, bony crest rising from the lumbar and 

 sacral vertebrae, consisting of their united neural or spinous processes. 

 The carapace rested directly on these bones, and was joined to them 

 by suture, as the roughened and expanded surfaces for such juncture 

 show. The entire union of the lumbar and sacral vertebrae into a 



* Birds with teeth. Third Ann. Rep. U. S. Geol. Surv., 1881-'82, p. 82. 

 t The gigantic extinct Armadillos and their peculiarities, with a restoration. Pop. 

 Science Monthly, XIII, pp. 139-145, 4 figs, in text. 



