224 PALEONTOLOGY OF NEW YORK. 



ANASTROPHIA, Hall. 1867. 



PLA.TB LXIU. 



Under the term Anastrophia has been grouped a number of species with 

 reversed convexity, the disparity of the two valves in this respect surpassing 

 that prevailing in Parastrophia. Tlieir external surface is covered with 

 numerous fine and sharp dichotomizing ribs, extending to the apices of the 

 beaks and frequently crossed by delicate concentric lines. The type of this 

 genus is the Pentamerus Verneuili, Hall, of the Lower Helderberg shaly lime- 

 stone, and with it are to be associated the Atrypa inlerplicata, (Sowerby) Hall, 

 and Anastrophia internascens, Hall, of the Niagara group ; the Terebratula deflexa, 

 Sowerby, of the Wenlock limestone, and the shell passing under the same name 

 in a corresponding fauna of the Island of Gotland. 



In this group again, the internal apophyses are subject to some variation. 

 The spondylia of the valves are quite large, extending not less than one-third 

 the length of the shell. As in Parastrophia, that of the pedicle-valve is the 

 wider and is supported by a median septum near its anterior extremity. In 

 the earlier species, A. internascens, A. deflexa, this spondylium may be thus sup- 

 ported for nearly or quite its entire length, but in A. Verneuili, the latest repre- 

 sentative of the group, the structure is usually the same as in later members of 

 Parastrophia. On Plate LXIII is given a figure of an interior of this species 

 in which the lateral walls of the spondylium have folded, one over the other, 

 and thus formed a tubular cavity open at both ends. What the significance of 

 this modification may be, can not be judged from the single specimen. In the 

 brachial valve the convergent plates generally rest upon the inner surface of 

 the shell, though at times, in A. Verneuili, the spondylium is supported at its 

 anterior extremity. The crural plates are extravagantly developed, forming 

 two broad wing-shaped vertical expansions, concave on their outer surfaces ; 

 their upper edges are curved over the hinge-line, their anterior edges broadly 

 notched, and below this point appears the base of attachment for the crura ; 

 the walls of the spondylium being connected with them at the most convex 

 point of their inner surfaces. The dental sockets are always small, and old 

 shells frequently show a false foramen in the beak, which is simply an exten- 



