178 IOWA GEOLOGICAL SURVEY 



proaoh to Dinichthys-like conditions, and a still further resem- 

 blance to the latter is observed in the form of the vomerine 

 teeth. The latter, if found in the detached state, might readily 

 be mistaken for the commonly so-called "premaxillary" teeth 

 of typical Dinichthyids. In addition, the dorsomedian and other 

 plates of the abdominal armoring are indistinguishable from 

 those of Dinichthys. 



The mandibles of this species, which are extremely well pre- 

 served in the holotype, bear a superficial resemblance to the 

 lower dental plates of Palseomylus, especially P. greenei, and 

 they are constructed more nearly after the pattern of Dinichthys 

 than of Mylostoma. Their anterior extremities are elevated 

 into a rather obtuse symphysial beak, which rises but little above 

 the broad, flat, deeply excavated functional surface. The latter 

 displays a single inconspicuous eminence or tubercle close to the 

 external margin, situated about midway the length of the oral 

 surface ; and at some distance behind this elevation is a second, 

 larger tubercle, rather elongate, and externally situated like the 

 first. This posterior prominence fits snugly against the single 

 large rounded boss of the opposing palato-ptyergoid dental 

 plate, thus determining the orientation of the latter with utmost 

 nicety, and affording a certain clue to the position of the cor- 

 responding element in Mylostoma. 



The splenial is developed as a long, slender shaft of bone, re- 

 sembling that of Dinichthys, but relatively deeper. In the type 

 specimen, preserved in the Yale Museum, both the right and left 

 elements are preserved in natural association with the articular 

 cartilage. This last has become more or less compressed through 

 fossilization, but remains attached to the outer face of the bony 

 shaft near its posterior extremity. There is no separate angu- 

 lare, nor dentary bone, the mandible being reduced to its lowest 

 terms and consisting, as is usual among Arthrodires, of merely 

 the splenial and dental plate.* The vomerine teeth are prehen- 



* Jaekel's claim to have discovered a well differentiated angulare in the mandible 

 of his so-called "Pholidosteus", in reality a synonym of Brachydirus, is discredited 

 by Dean (Science, 1908, 27, p. 203), who suggests that the structure in question 

 may be a displaced portion of the well known interlateral plate. Jaekel's supposed 

 articular element Dean likewise interprets as a detached portion of the antero- 

 ventro-lateral plate. It is indeed noteworthy that the former of these alleged man- 

 dibular components shows a strongly tuberculated surface, which is scarcely recon- 

 cilable with its assignment by Jaekel to an internal position. 



