CYRTINA 287 



1908. Cyrtina acutirostris Rowley, Mo. Bureau Geol. and Mines, vol. 8, 

 2nd ser., p. 84, pi. 18, figs. 16-20; p. 87, pi. 19, fig. 2. 



Description. Shell small, subpyramidal in form, broader than long, the 

 greatest width along the hinge-line, the cardinal extremities angular. The 

 dimensions of two examples are : length of pedicle valve 13.4 mm. and 

 7.3 mm., length of brachial valve 8.2 mm. and 6.2 mm., width 15.6 mm. and 

 11 mm., thickness 10 mm. and 6.9 mm., height of cardinal area 5.7 mm. 

 and 5.1 mm. 



Pedicle valve subpyramidal with the apex nearly erect or curved to- 

 wards the hinge-line in various degrees ; the surfaces of the lateral slopes 

 unusually convex throughout and sloping steeply from the umbo to the 

 lateral and antero-lateral margins, sometimes a little compressed along 

 the cardinal margin towards the cardinal extremities ; mesial sinus shal- 

 low or of moderate depth, originating at the beak, rounded or subangular 

 in the bottom ; the cardinal area high, nearly flat in the younger shells and 

 lying at an angle of from 60 to 90 degrees to the plane of the valve, re- 

 maining nearly flat or becoming more or less strongly arched in full grown 

 shells, the lateral margins are sharply defined and angular, and slope in 

 nearly straight lines or are gently convex from the beak to the cardinal 

 extremities; the delthyrium is high and narrow and is covered by a con- 

 vex pseudodeltidium which is pierced by an elongate, narrow foramen 

 sometimes reaching from the apex more than half way to the cardinal 

 margin ; each lateral slope is marked by three or four simple plications 

 originating along the cardinal margin, the two bounding the mesial sinus 

 are much the strongest, the others rapidly becoming much narrower and 

 weaker. Internally a strong median septum reaches from the beak half 

 way to the front margin of the valve, towards the cardinal area the septum 

 is flanked by the rather short dental plates which reach from the margins 

 of the delthyrium; in the angle formed by the junction of the dental 

 plates with the median septum an angular ridge extends from the apex of 

 the delthyrium to the free margin of the septum and in front of this ridge 

 between the laminas of the septum is a pair of laterally compressed canals 

 placed side by side and extending from near the apex of the valve to the 

 free margin of the septum ; the muscular scars are feebly developed, and 

 the muscles were probably attached in large part to the sides of the 

 septum. 



Brachial valve depressed convex, very shallow, compressed towards the 

 cardinal extremities, the greatest convexity near the middle ; the mesial 

 fold low or sometimes moderately elevated as it approaches the front 

 margin, non-plicate or with a slight mesial depression which is sometimes 

 present only towards the beak ; each lateral slope marked by three or four 

 plications, the first of which adjacent to the mesial fold is nearly as broad 

 as the fold itself, the others much smaller, the last one often being nearly 



