OBOLID^. 427 



in an elongated shell (PI. XXXII, fig. 5e). The approximation in position of the main vascular 

 trunks is an tmusual feature but one that, o\ving to the state of preservation of the material, is 

 not altogether satisfactorily determined. 



Observation. — This is a most interesting species and it is to be regretted that better material 

 has not been found to illustrate the interior of the valves. The outer surface at once suggests 

 comparison with Oholus (Acritis) antiquissimus (Eichwald) (PI. XIII), but that species has a 

 strong convex form and a short visceral cavity that is peculiar to it and that caused Mickwitz 

 [1896, p. 205] to retaia Acritis as a subgenus. The size and position of the visceral cavity is 

 normal for the genus. Matthew [1899, PL II, fig. 2e] gives a diagrammatic figure of the dorsal 

 valve in which he places large muscle scars far forward into the valve. A careful study of his 

 specmien shows that the vascular canal in front of the anterior lateral scars bifurcates about 

 the center of the shell. Another specimen laterally compressed (PL XXXII, fig. 5e) shows the 

 central muscle scars in about the center of the shell despite the distortion of the shell. The 

 positions given them by Matthew [1899, PL II] in the broad form of the shell appear to have 

 been decided by their position in the compressed elongated shell; the same is also true of the 

 strong median ridge in his figure 2e. 



Matthew [1899, p. 201] proposes the subgenus Palxobolus for this species on accoimt of the 

 "close approximation of the vascular trunks" on the ventral valve. Tliis character is shown 

 in shells elongated and compressed laterally, but even by making allowance for compression 

 they are closer than in other forms of the genus. 



The specific name is derived from Cape Breton. 



Formation and locality. — Middle Cambrian: (13d') Sandstones opposite the third waterfall in Dugald Brook, 

 between divisions E2a and E2b; (lOp) sandstones just below the waterfall in Divisioli E2b; (131' and 344i [Matthew, 

 1903, p. 143]) sandy shales of Division E3a; and (13n" and 344b [Matthew, 1903, p. 142])<^ Sandy shales of Division 

 JESd; all in Matthew's [1903, p. 21] Etcheminian, on Dugald Brook, Indian River, Cape Breton, Nova Scotia. 



(344c [Matthew, 1903, p. 142]) Sandy shales of Division E3d of Matthew's Etcheminian, on Gregwa Brook, Indian 

 Hiver valley; (lOp") sandstone on the small brook on the hill between the bridge over Indian River and McPhees 

 Brook; and (lOr) arenaceous shales of Division C3a? of Matthew at McAdam Shore, East Bay, east of Bras d'Or Lake; 

 all in eastern Cape Breton, Nova Scotia. 



Obolus (Palxobolus) bretonensis lens (Matthew). 



Oholus lens-primus Matthew, 1902, Trans. Roy. Soc. Canada for 1902, 2d ser., vol. 8, sec. 4, No. 3, pp. 94-9.5. (Char- 

 acterized. This is the first reference to this variety.) 



Obolus lens Matthew, 1902, idem, p. 9-5, PI. I, figs. 6a-f. (Characterized.) 



Obolus (Palxobolus) lens Matthew, 1903, Geol. Survey Canada, Rept. Cambrian Rocks Cape Breton, pp. 144-146, 

 PI. X, figs. la-f. (Described and discussed. Figs, la-f are copied from figs. 6a-f, respectively, of the preced- 

 ing reference.) 



Obolus (Palseobolus) lens longus Matthew, 1903, idem, pp. 146-147, PL VII, figs. 4a-b. (Described and discussed as 

 a new variety.) 



Matthew [1902b, p. 95] gives a specific value to the difference in the number and closeness 

 of the concentric striae on the surface of the shell, but from a study of the types of Obolus {Palx- 

 cbolus) hretonensis and material representing the surface of 0. (P.) lens I do not think lens is 

 more than a variety of bretonensis. The variety longus is based on specimens that have been 

 compressed laterally to such an extent as to lose their original outline. With the types before 

 me I do not feel justified in assigning them as a variety distinct from the variety lens -with 

 which tliey are associated. 0. (P.) bretonensis occurs in beds between layers containing the 

 variety lens. 



Loper collected a number of specimens of a shell that may be referred to the variety lens 

 in Matthew's assise 2c, in which the concentric striae, or ridges are very fine on the umbo, grad- 

 ually becoming coarser toward the front, where they are of the size of the average in the typical 

 forms of 0. (P.) bretonensis. These shells also vary in outline so as to include the variety 

 longus. In a layer just below that containing these shells he found a single shell with concentric 

 ridges stronger and coarser than the average of 0. (P.) bretonensis. 



'■344b is the type locality. 



