Features of 

 tlif interior. 



Sculpture of 

 the surface. 



212 



There is little to add to the description of the ventral valve, but the 

 small size of the space occupied by the muscles is remarkable. The height 

 of the cardinal area is only about one-eight of the length of the valves, 

 which is not the case in Obolus. 



A mould of the interior of the dorsal valve from McLeod brook, shows 

 much more distinctly than those from the St. J ohn basin the nature of 

 the impressions of the central muscles ; and the crowding together of 

 these scars, corresponding to the description of Monobolina by Salter. 

 Although there is sometimes a faint elevation on the median line in the 

 anterior of the valve, the median ridge cannot be said to extend beyond 

 the mid-length of the valve, and it is bordered by two deep and narrow 

 grooves in the anterior half of its length, that separate the print of the 

 anterior adductors, which in their turn are bordered outside by a shallow 

 groove, as represented by Davidson for Monobolina plumbea. The poste- 

 rior half of the mesian ridge is much fainter than the anterior ; in fact 

 the posterior half of it is almost obsolete. 



The position of the central group of muscles in the dorsal valve of this 

 species (M. re/ulgens) as well as the short visceral callus of the ventral 

 valve, appear to exclude this species from Obolus, and show its rieareness 

 to, or identity with Monobolina. Moreover, the weak muscle scars, and 

 the absence of an area to the dorsal valve, are also not found in Obolus. 



Sculpture (2).-^-The state of preservation of the examples from Cape 

 Breton, give a much better knowledge of the surface characters of this 

 species, than those from Navy island. The surface was spoken of as 

 " brilliant ", but this brilliancy is found on the second layer of the shell ; 

 the Cape Breton examples appear to have a thin outer prismatic layer of 

 mineral matter, which (if not an accidental addition) is a dull surfaced 

 outer layer, which when viewed with a strong lens exhibits a finely gra- 

 nular surface, ornamented with concentric ridges ; these ridges are low, 

 often interrupted or broken, and separated by smooth granulated spaces. 

 The under corneous layer is aho marked by concentric undulations of 

 growth, and towards the front of the shell, and especially on the inner 

 surface of this layer, by numerous sub-parallel lines, radiating from the 

 umbo. There is still another layer of the shell which forms its inner sur- 

 face, and on which the muscle scars and vascular trunks are impressed. 



I more than suspect that the surface which Mr. Salter described as 

 that of Obolella (Monobolina) plumbea is the corneous surface of that 

 species, corresponding to the similar one in this.* 



* Mem. Geol. Surv., G. Britain, vol. iii, p. 539, London, 1881. 



