28-1: CONTRIBUTIONS TO CANADIAN PALAEONTOLOGY. 



and central region tumid, its umbo rather prominent, and its beak incur- 

 ved and slightly recurved. 



Surface markings consisting of fine, subequal and closely disposed, radi- 

 ating raised lines, which increase in number by bifurcation, trifurcation 

 and intercalation, and are crossed by a few irregularly disposed, Vmt for 

 the most pai-t distant, concentric stria^ of growth. Around the outer 

 margin of the dorsal valve figured (tig. 3), about 136 radiating raised 

 lines can be counted, and in a still larger dorsal valve, which is not figured, 

 about 150 were counted. 



Three casts of the interior of the dorsal \-alve of a shell which is pro- 

 bably refei-able to this species, from the local base of the Middle Devonian 

 at Devils Point, Lake Winnipegosis (figs. 5 and 5 a) shew two linear 

 grooves, which probably represent the impi'essions made by the brachial 

 processes, diverging on each side of the umbo, also a minute and very 

 short slit, which widens into a small subciroular or subrhomboidal per- 

 foration in the apex of the beak. Muscular impressions not clearly defined 

 in any of the specimens collected. 



Of the two testiferous specimens figured, the one showing the dorsal 

 valve only (fig. 3) is thirteen millimetres long and si.xteen broad, while 

 the one drawn to shew the front margin (fig. 4) and relative convexity of 

 the two valves, is nineteen mm. broad and ele\'en mm. and a half in depth 

 or thickness, though it is too imperfect posteriorly to show the exact 

 length. 



Dawson Bay, Lake Winnipegosis, on the east side, at Whitea\'es Point, 

 and on two small islands in its immediate vicinity ; on a small island 

 aljout three miles north of Salt Point, and, on the west side, at the first 

 small point north of the mouth of the lied Deer IJivcr, J. B. Tyrrell and 

 D. B. Dowling, lcS89 : one to three specimens from each of these localities, 

 most of the former being detached dorsal valves, the only example with 

 Ijoth valves preserved being very immature. 



The sjjecimens for which the foregoing provisional name is suggested 

 may prove to be only a small local variety of of the typical 0. Iniveiisin of 

 Hall, as originally described and figured in the Geology of Iowa, but not 

 of the large northern form which has been referred to that species. They 

 seem to differ from the typical 0. Joivi'iisin chiefly in their much smaller 

 size and more quadrangular form ; also, though this may be due to their 

 being highly dolomitized, in the apparent absence of the "tubular open- 

 ings " on the surface of the radiating stria=, and of " fine pores or puncta; 

 over the entire surface," which are said to be characteristic of 0. lowensis. 



