326 PALEONTOLOGY OF NEW YORK. 



better means for the arrangement and simplification of this group can now be 

 offered. Some of the divisions made by the French authors must be aban- 

 doned, since the forms on which they were founded have been advanced to a 

 generic or subgeneric designation ; namely, the group Laves, de Koninck, which 

 included P. Leonhardi = Koninckina ; the Proboscidei, de Koninck, for shells like 

 P. proboscideus, de Verneuil, a group for which (Ehlert has proposed the sub- 

 generic term Proboscidella ; and the Caperati, de Verneuil, a group of Devo- 

 nian and Carboniferous forms essentially equivalent to Productella. Waagen 

 has proposed to divide the group Striati, de Verneuil, into the Lineati and 

 Irregulares. 



We have then the following arrangement, with citations of American repre- 

 sentatives of each group : 



I. Lineati, Waagen. Surface covered with fine radiating costae which are 

 rarely spinous and are not crossed by concentric plications or wrinkles. These 

 shells are greatly produced and sometimes the anterior margins of the valves 

 are modified by the development of a fold or sinus (P. Americanus, Swallow, = 

 ? P. equiradiata, Shumard). The shells were very fragile and have usually been 

 subjected to much distortion in fossilization. 



Examples : 



P. Cora, d'Orbigny. P. Ame?icanus, Swallow. 



P. Prattenanus, Norwood. P. (Rquicostatus, Shumard. 



P. Icevicostus, White. P. pileiformis, Newberry. 



P. ovatus, Hall. P. nodosus, Newberry. 



II. Irregulares, Waagen. Elongate shells very narrow at the beak, mytili- 

 form in outline; mode of growth quite irregular. Surface as in the Lineati; 

 spines grouped almost wholly about the cardinal line. 



We are not aware that any member of this gi'oup has been recognized in 

 America. The best known species is P. striatus, Fischer, a widely distributed 

 form in the Carboniferous limestone of Europe. Waagen has described the 

 species P. compressus and P. mytiloides, from the Permo-Carboniferous of the 

 Salt-Range. 



III. Semireticulati, de Verneuil. The longitudinal ribs are sparsely spinous ; 

 surface of the visceral disc covered with concentric wrinkles. 



This group includes the greater number of North American species, but it 



