CRANIA 45 



elevated above the inner surface of the valve, the posterior pair situated 

 just within the raised marginal border and rather widely separated, much 

 less elevated than the anterior pair. 



Brachial valve apparently wanting from all the specimens seen. 



Remarks. This species is known only from the types which consist of 

 five individuals attached to one shell of Spirifer grimesi. The species 

 may be recognized by its size, the raised marginal border of the pedicle 

 valve and the conspicuously raised anterior muscular scars. 



Horizon. Burlington limestone. 



CRANIA CHESTERENSIS Miller and Gurley 

 Plate I, Figs. 27-29 



1897. Crania chesterensis Miller and Gurley, Bull. No. 12, 111. State Mus. 

 Nat. Hist., p. 47, pi. 3, figs. 24-26. 



Description. Shell small, gregarious in habit, subcircular in outline. 

 The dimensions of one of the type specimens are : greater diameter 8 mm., 

 lesser diameter 7 mm. 



Pedicle valve, which is firmly cemented to external objects, not 

 observed. 



Brachial valve depressed, thin except at the apex and about the outer 

 margin, the apex subcentral; in the condition of preservation of all the 

 specimens examined, the thinner portion of the valve between the apex 

 and the periphery, appears to be collapsed or depressed, leaving a distinct- 

 ly elevated marginal rim and a node-like subcentral elevation at the apex. 

 Surface marked only by sublamellose, concentric lines of growth which 

 are somewhat crowded near the periphery but are much less conspicuous 

 upon the collapsed portion of the valve and towards the apex. 



Remarks. This little species grows most commonly upon the thickened 

 borders of species of the bryozoan Lyropora, but it also occurs sometimes 

 upon the fenestrated portion of the same bryozoan, and a single specimen 

 has been observed attached to the shell of a Spirifer. The species can be 

 easily recognized, as it is commonly preserved, by the raised peripheral 

 ring, and the raised central node. Whether the raised peripheral ring is 

 due to the thickening of the brachial valve at the margin, as has been 

 suggested in the above description, or whether it is due to the presence of 

 a raised or thickened margin of the under or pedicle valve, cannot be cer- 

 tainly determined from the specimens, but perhaps the latter supposition 

 is most probable, in which case the entire brachial valve is thin except at 

 its apex. 



Horizon. Chester group. 



