ETCHEMINIAN FAUNA OP CAPE BRETON. 201 



Sculpture. — Somewhat obscure fine concentric stria}, visible with 

 a lens, are present on all parts of the valves. A very fine granulation 

 also can be seen. The outer layer of the shell, which carries the 

 ornamentation, is calcareous. 



Size. — Length 5 mm., width, 3| mm. 



This species is of about the same size and geological age as 

 Lingulella ferruginea, Salter ; but if Davidson's figure is correct chat 

 species had a hinge area twice as long as this. It may be further 

 observed that Davidson has included in this species forms from much 

 higher zones of the Cambrian (Dolgelly group, etc.), but the char- 

 acters are so vague that any small oval species might be referred to 

 L. ferruginea. We are subject to the dilemma of choosing between 

 two or several species which by form and size represent L. ferruginea, 

 but which by other characters are found to be distinct from each 

 other ; hence, if one is L. ferruginea, the others are not. 



Lingulella Granvillensis, Walcott, of the New York Cambrian, is 

 of about the same size as this, and approaches it in form, but the 

 moulding of the interior of the dorsal valve differs. 



Pal^eobolus, n. sub-gen. 



Distinguished from Obolus proper by the close approximation of 

 the vascular trunks, as shown by their impression on the ventral 

 valve, and by the forward direction of its branches. The callus of 

 the visceral cavity of this valve is correspondingly narrow (therefore 

 the muscle scars are also approximated). Yet the valve is round as 

 in Obolus. 



In default of more exact criterea, we have adopted here and 

 elsewhere the following characters for distinguishing Obolus from 

 Lingulella, — roundness of outline, short cardinal area and depressed 

 beaks, advanced position of muscle scars in the valves, and strong 

 arch of the vascular trunks in the ventral as well as the dorsal 

 valve. There is, however, a more important distinction, which, in 

 consequence of imperfect preservation of the valves, can seldom be 

 observed, that is, the position of the secondary muscles of the central 

 group in the ventral valve, as compared with the great muscle of that 

 group. In Obolus they are lateral, but in Lingulella anterior to the 

 great muscle. This shows a radical difference of structure between 

 the two genera. It will be observed that the relation of these muscles 

 is as yet unknown in Palpeobolus. 



