312 PALJEONTOLOGY OF NEW YORK. 



behind them are linear depressions or dental sockets. There is also a low median 

 ridge extending from the base of the cardinal process into the pallial region. 



The substance of the shell shows a coarsely prismatic cellular structure, as in 

 PoRAMBONiTES and EiCHWALDtA. According to Barrois, this cellular lamina is 

 not superficial but is covered by a thin epidermal layer. 



Type, Aulacorhynchus Par.hii, Dittmar. Carboniferous limestone. 



Observations. There is still much to be learned of the structure of these 

 curious shells. Their similarity in external aspect to De Koninck's Chonetes 

 concerUrica, led some of the early writers to refer them to that genus and species, 

 but Semenow, the first author to notice these fossils, observed their differ- 

 ences from Chonetes, in the absence of cardinal spines and the existence of a 

 thickened triangular plate in the pedicle-valve, and suggested that they were 

 to be regarded as typical of a new brachiopod genus. Meek and Worthen are 

 the only authors who have described the brachial valve, and upon it was based 

 the conception of their genus Isogramma, which must yield to Dittmar's term 

 introduced two years earlier. More recently Barrois has added some important 

 observations upon the structure of the genus. 



Species of this genus are not common, but appear to be widely distributed in 

 Carboniferous countries, Russia, Silesia, Scotland, and the Asturias. In North 

 America the only species known is the Isogramma millepundata, Meek and 

 Worthen, from the upper Coal Measures of Marion county, Illinois. 



The origin and affiliations of Aulacorhynchus are involved in great uncer- 

 tainty. The resemblance to Chonetes is fortified by the existence of a stout 

 cardinal process, while the triangular muscular plate, the close incurvature of 

 the beak and obscuration or obliteration of the pedicle-passage and deltidium, 

 are features similar to those existing in Eichwaldia. It may be suggested that 

 the pedicle in Aulacorhvnchus was extruded in a manner similar to that in 

 Eichwaldia, and that, hence, the platform may have been vaulted and slightly 

 raised above the bottom of the valve, though this is not evident from the usual 

 preservation of the fragile shells, where compression has closed any such cavity. 

 These similarities to Eichwaldia are still further seen in the coarse cellular 

 structure of the shell. 



