THE OTTAWA NATURALIST--*-^ 



VOL. XXIX. NOVEMBER, 1915 No. 8 



A NEW ORDOVICIAN PELECYPOD FROM THE OTTAWA 



DISTRICT.* 



By Alice E. Wilson. 



The shell is of medium size and subelliptical in outline, 

 length and height about as 2:5. The valves are very slightly 

 convex. The cardinal margin is straight posterior to the beaks 

 for about two-thirds the length of the shell, making an angle of 

 45° with the anterior margin, which continues as a straight line 

 nearly to the median transverse axis of the shell, thence curving 

 into the anterior and basal margins. The latter margin bends 

 slightly upward opposite the broad weakly-defined sinus. The 

 posterior end is slightly trtmcated obliquely, but joins the basal 

 margin with a moderately narrow curve. The anterior margin 

 and Lhe straight cardinal margin form a more obtuse angle than 

 that f the posterior end, and the curve with which it joins the 

 vent '. margin is less narrow. There is a slight constriction 

 benea uh the very moderately raised umbones. The lunule , which 

 is evidently very narrow, is partially destroyed on the specimen 

 examined. The sinus is very shallow, moderately broad and less 

 oblique than most other species of this genus. The umbonal 

 ridges are not prominent, and become imperceptible in the pos- 

 terior portion of the shell, which is almost fiat. Anterior to the 

 sinus there is a slight infiatation. The concentric growth lines 

 are very tine, but anteriorly they are gathered into about a dozen 

 strong ridges, which end abruptly in the oblique cardinal margin. 

 Posteriorly the ridges of growth lines almost disappear. 



The most striking characteristic of the species, however, 

 is the unique marking. A series of fine granules crosses the con- 

 centric growth lines, radiating from the umbonal region. Near 

 the beak they are very fine, hardly visible to the naked eye, but 

 they become much stronger away from it, so that in the ventral 

 half of the shell they have almost obliterated the concentric 

 growth lines, except anteriorly where the strong ridges of con- 

 centric growth lines are still prominent. On the dorsal half of 

 the posterior portion of the shell there is a still more complex 

 marking. In addition to the very fine concentric growth lines 

 crossed by the radiating series of granules, which here are very 

 minute, there is a very fine double network of lines running 

 obliquely from granule to granule, forming a regular mesh, with 



♦Published by permission of the Deputy Minister of Mines. 



