Cope.l 10 [April 17, 



I described from specimens procured by Professor Newberry, in the 

 coal measures of Linton, Ohio.* The peculiar structure of the tarsus 

 is identical as to the number of its elements, and the other characters 

 agree in general. There are important differences also, which would 

 refer Ichthycanthus to another family. Thus the dorsal vertebrae have the 

 centra deeper than long, and the ribs are free. In the absence of the 

 skull, it is not possible to be sure as to which of the classes, Reptilia and 

 Batrachia, these genera represent. 



Another form presents some important points of resemblance ; that is 

 the genus Mesosaurus of Gervais.f The M. tenuidens Gerv. was brought 

 by Verreaux from an undetermined formation of Griqualand, South 

 Africa. The specimen, like that of the Stereosternum tumidum, is exposed 

 on a slab, and embraces only the head, neck, thorax and anterior limbs. 

 As the dorsal vertebra? are obscured by matrix the only point in which actual 

 comparison can be made is the ribs. These are quite identical in the two 

 types, but the articulations with the vertebral centra are invisible in the 

 Mesosaurus. There are apparently impressions of abdominal dermal rib- 

 lets, but they are suspected by Gervais to be the tracks of Annelids. Ger- 

 vais thinks the skull has but a single condyle. The scapular arch consists 

 of coossified scapula and coracoid, but clavicle, prsesternum and sternum 

 are not visible. The coracoid is different in form from that of Stereoster- 

 num. The humerus is, on the other hand, almost identical, and the carpus 

 is nearly what one would expect to find in the Brazilian form. There are 

 in the first carpal row, two large bones, and in the second, four small 

 ones. 



Habits. — The structure of the limb articulations and those of the ele- 

 ments of the posterior foot show that this was a genus of aquatic habits. 

 The firm attachment of the ribs shows further that this type had no inter- 

 costal respiration, but used its sublingual or its abdominal muscles, or 

 both, in the act of inhaling air. We may suppose that in its aquatic habi- 

 tat it retained air in the lungs for considerable periods, and only respired 

 on reaching the surface of the water ; or later investigation may show 

 that it is branchiate. 



Geological position. — The peculiar characters of this form and the diffi- 

 culty of determining its true position in the system, present an obstacle 

 to the interpretation of its probable geological age. It has a good many 

 resemblances to the suborder Choristodera of the order Khynchoce- 

 phalia (represented by the Champsosauridse). This type first appears in 

 the Laramie or latest Cretaceous, and continues only to the top of the 

 lower Eocene. The order Rhynchocephalia is an unsatisfactory one for 

 geological purposes. It still exists in one genus, the llatteria of New Zea- 

 land, and may have existed in the Trias ; although this is not certain. 



Prof. Derby informs me that some specimens of Schizodus have been 

 found in the same beds, and he therefore iufers that their age may belong 



♦Proceedings Amer. Philosoph. Society, 1887, p. 573. 

 tUeuerul Zoology and Paleontology. 



