1887.] -^11 [Cope. 



Future comparison must be bad witb the genus Aetosaurus Fraas,* which 

 accompanies Belodon in the Upper Keuper of Wurtemberg. That genus 

 is encased in parallelogrammic scuta arranged in contiguous cross-bands, 

 on both surfaces of the body. But the scuta are not co-extensive with 

 tlie ribs as in Typotliorax ; at least tlie latter are represented by Fraas as 

 much narrower than the osseous dermal bands. The latter are also trans, 

 versely subdivided in Aetosaurus. 



It is highly probable, however, that Typothorax represent Aetosaurus 

 in the Upper Trias of North America, and may belong to the same family 

 of the order Rhynchocephalia. 



The relations of the dermal bones and ribs are highly interesting. The 

 great expansion of the latter needs but the development of sutural sur- 

 fivces on their borders to produce an osseous continuum. The same modi- 

 fication of the dermal bones above it would form a second external 

 roof. A subsequent fusion of the superior and inferior roofs would give 

 us the testudinate carapace. And this history would be what embryology 

 teaches us is the origin of that remarkable modification of the dermal and 

 true skeleton exhibited by that order of reptiles. It is probable that Typo- 

 thorax is nearly allied to the type from which the order of tortoises has 

 been derived. It is unfortunate that we know nothing of its skull and 

 vertebrae, but there is nothing in the characters of the femora to preclude 

 the above hypothesis. They belong to a type which progressed in a 

 prone position, and which probably differed much from both Belodonts 

 and other Dinosauria. 



Char. Specif. Ribs strongly convex in the longitudinal direction ; in the 

 transverse direction flat above, and with a longitudinal convexity below. 

 This convexity occupies about one-third of the inferior surface, and ex- 

 tends obliquely to one of the lateral borders at the extremity. At the 

 other extremity the surface is flat, the rib-convexity disappearing. la 

 both ribs one edge is subacute and the other obtuse. The rib-thickening 

 runs out to the thin edge. The dermal scuta have the same width as the 

 ribs. They have thicker and thinner edges corresponding with those of 

 the ribs. Where the rib-thickening of the latter is prominent, the dermal 

 bone has a median convexity below ; and this disappears at the other end 

 as the thickening does from the rib. The superior face of the dermal 

 bones is perfectly flat. It is sculptured with coarse shallow pits, separated 

 by obtuse ridges, which have a reticulate pattern, since the pits are not 

 in rows. 



These osseous bands are probably in contact, thus forming an impene- 

 trable buckler, as in Aetosaurus. One edge of the osseous combination 

 of rib and dermal plate gapes, the thin edges of the two elements diverg- 

 ing so as to receive the margin of the adjacent band. The matrix along 

 this border is clearly impressed so as to prove the former presence of the 

 succeeding portion of the carapace. 



* Aetosaurus ferraius Fraas ; Festschrift zur Feier d. vierliundertjahrigeii Jubiliiums 

 Univ. Tubingen, 1877. 



