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 ©n Plates IY h and IY 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. {Cramelia) ffamiltonm is shown upon Plate IV i to 
bear the surface-characters of Tropidoleptus carinatus, Spirifer audaculus, Microdon 
{Cypricardella) 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 striae of those shells, Mr. Ulrich has 
given the name C. parallela. Less frequently the same species attaches itself 
to gastropod shells of the genera Pleurotomaria, 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. socialis, Ulrich, and C. mul- 
tipunctata, 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. 
