144 PALEONTOLOGY OF NEW YORK. 



It is also frequently observed that attached shells from which the upper 

 portion of the brachial valve has been broken, show the lower valve to have 

 slipped out of its normal position without being able to escape from the inter- 

 nal cavity of the shell. Furthermore, the Schizocrania filosa of the Utica slate 

 of New York is usually found free, and of thirteen examples before us of S. 

 Schncherti, none are attached; which facts together induce us to believe that a 

 parasitic habit requiring the modification of either valve to any such degree as 

 in Crania, can not properly be considered a character of the genus. 



The transverse plate in the apex of the notch in the pedicle-valve is directly 

 comparable to the similar structure in (EhlerteUa pleurites, and is undoubtedly 

 homologous with the external pedicle-groove seen in Orbiculoidea and Schizo- 

 TRETA. In Schizocrania it has formed a sui-face over which the pedicle passed, 

 increasing in size with age and becoming a conspicuous feature in old shells. 

 The plate is not elevated or depressed above the plane of the valve as it is in 

 CE. pleurites, but it shows the median ridge projecting at the margin, a feature 

 which is strongly marked in the latter species. (See discussion of ffiHLERTELLA 

 on pages 120, et seq.) 



In addition to tlie two species of the genus mentioned as occurring in the 

 fiuna of the Hudson group, are two other species in the Lower Helder- 

 berg fauna, less completely known but giving indications of structure similar 

 to that of S. filosa. These are the form described in the Supplement to this 

 volume as S. (?) Helderbergia, sp. nov., and that mentioned by Barrett in the 

 citation above given as Trematis {Schizocrania) superincreta. The species de- 

 scribed as Trematis rudis. Hall, from the Trenton limestone at Clifton, Tennessee, 

 is also imperfectly known, but may prove a Schizocrania. 



