146 PALEONTOLOGY OF NEW YORK. 



1884. Crania, White. Thii-toeiith Kept. State Geologist Indiana, p. 121. 



1884. Crania, Spexoer. Bnll. No. 1, Mus. Univ. State of Missouri, p. 57. 



1886. Crania. Rtxouekkrr. Bull. Buffalo Soc Nat. History, vol. v, pp. Iti, 17. 



1889. Crania'?, Walcott. Proceeding-s United States National Museum, 1888, p. 441 ; Advance sheets. 



1889. Crania, Beecher and Ci..\rke. Memoirs N. Y. State Museum, vol. i, No. 1, p. 13. 



1889. Crania, Nettelkoth. Kentucky Fossil Shells, p. 2. 



Diagnosis. Shell inequivalve, inarticulated, without perforation for a pedicle ; 

 subcircular in outline, generally somewhat transverse across the posterior mar- 

 gin ; attached by the apex or the entire surface of the lower valve. Ventral 

 or lower valve depressed-conical or conforming to the surface to which it is 

 attached. Dorsal or upper valve more or less conical with a subcentral, poste- 

 riorly directed apex. External surface of the valves usually smooth, sometimes 

 spinose or with concentric or radiating strise. In the interior of both valves 

 are two pairs of large adductor scars, the posterior of which are close upon the 

 margin and widely separated, the anterior near the center of the shell and 

 close together, more approximate in the lower than in the upper valve. These 

 posterior scars are often strongly elevated on a central callosity which sur- 

 rounds their anterior margins. The margin of the lower valve is usually broad 

 and thickened. Impressions of the pallial genital canals coarsely digitate. 



Shell-substance calcareous ; strongly punctated by vertical canals which be- 

 come subdivided toward the epidermal surface. 



Type, Crania craniolaris, Linne. 



Observations. Crania is remarkable for presenting an association of shell- 

 characters which have undergone no essential change from the earliest known 

 appearance of the genus in Lower Silurian faunas to the present. Indeed 

 neither paleozoic nor recent species indicate material variation from the type 

 of internal structure found in C. craniolaris, while certain Mesozoic forms (C. 

 Parisiensis, Defrance, from the Jurassic, C. tripartita, Miinster, of the Cretaceous, 

 etc.), give evidence of so great departure from the type in the development of 

 internal diverging septa, in one or both valves, that separate subdivisions have 

 been established for their reception; viz., Ancistrocrania and Craniscus, Dall, 

 respectively. 



The degree of attachment of the lower valve has been made a basis of sub- 



