148 PALEONTOLOGY OF NEW YORK. 



which, according to Hancock, have both their extremities attached to the same 

 valve (dorsal) ; mm is the median or mesenteric muscle, probably acting as a 

 support to the viscera. Mr. Davidson has given figures of a dorsal valve of 

 Crania (Pseudocrania) divaricata, in which these minor impressions are shown,* 

 and they may be seen more or less distinctly on some of the accompanying 

 figures on Plates IV h and IV i. 



The external surface of the upper valve in palaeozoic Cranias is either smooth, 

 that is with only the concentric lamellose growth-lines; covered with radiating, 

 elevated, frequently dichotomizing costae ; or minutely spiniferous. In the first 

 mentioned condition the surface is very generally modified by the contour of 

 the body to which the lower valve is attached, and in most of these smooth 

 species there appears to have been great indifference as to the zoological nature 

 of the host. For example, C. {Craniella) Hamiltmm is shown upon Plate IVi to 

 bear the surface-characters of Tropidoleptus carinatus, Spirifer audaculus, Microdon 

 {Cijpricardella) bellistriatus, and it is often found on other species of mollusca, 

 and rarely upon trilobites and corals. Among the smooth forms occurring 

 in the Hudson fauna in Ohio and Kentucky^ are several to which various 

 specific names have been applied in accordance with the modification of the 

 surface from attachment to different hosts; C. scabiosa, Hall, often with a 

 considerable degree of convexity and a smooth exterior, usually adheres to 

 Strophomena alternata, or some species of Orthis, and to individuals having the 

 parallel markings corresponding to the strise of those shells, Mr. Ulrich h;is 

 given the name C. parallela. Less frequently the same species attaches itself 

 to gastropod shells of the genera Pleurotomauia, Cyclonema, etc., or to small 

 crinoid stems, or to the surface of monticuliporoid corals, undergoing in each 

 case just such modifications as the circumstances have required. These varia- 

 tions pass under the names C. percarinata, Ulrich, C. sodalis, Ulrich, and C. mul- 

 tipunciata. Miller, a subdivision wholly based on accidental characters. Indeed, 

 among most of the smooth species in palaeozoic faunas, unless there is some 

 evident difference in interior character, a specific designation can hardly serve 

 a broader purpose than to indicate a different association. The plicate or 



* British Silurian Brachiopoda, pi. viii, figs. 11, 11 a, 12 a. 



