102 



AXNAI.S OF THE CaRNHGIK MuSEUM. 



Developmental Changes. 



The two varieties have practically the same development, as the 

 only difference is in the time of the appearance of the new striae. 

 They will therefore be considered together. In the adult the two 

 can be separated only under the most favorable circumstances. In 

 well-preserved specimens the variety arctistriatus shows a surface 

 thickly covered with sharp, subequal stride. O. pectenacea has slightly 

 wider spaces between the stris and certain of the strire are stronger 

 than others. With young specimens, especially if the dorsal valves 

 are at hand, there is no difficulty in separating them. The variety 

 pectenacea has from 15 to 19 strong elevated stride between which are 

 lower interspaces containing one or three strioe. In the variety (i';r- 

 tistriatii s i\\e. sir'rx. a.r& so crowded together that this alternating appear- 

 ance is not obtained. 



Nepionic Stage. — The smallest shell in the collection is a dorsal 

 valve .71 mm. in length and .80 mm. wide. It is almost circular in 

 outline, slightly convex, and is smooth for half the distance to the 

 front. There are fifteen sharp striae, all originating at the same time. 



Fit;. 18. OrtholJictes chevmngensis Conrad ; specimen retaining both valves, 

 and showing pedicle tube and primary plications. X ^^■ 



Fk;. 19. Ventral view of the same specimen. X I^. 



Fk;. 20. The same species. Cardinal view of another specimen showing pedicle 

 tube and relative convexity of valves. X 32- 



The nepionic stage in this species is of short duration. On all the 

 well preserved dorsal valves the smooth initial shell can be seen. It 

 is almost exactly circular, convex, and from .35 to .43 mm. in 

 diameter, usually about the lower limit. 



The smallest ventral valve is on a specimen which retains both 

 valves. It is .73 mm. long and i.i mm. wide with a width of i mm. 

 at the hinge. It is subelliptical in outline, convex, with a high 

 cardinal area. The delthyrium is almost completely closed by a 

 strong convex deltidium, which at the apex is prolonged slightly, 

 forming an exsert tube which is perforated for the passage of the 

 pedicle. On this specimen there are 13 stri;e on the dorsal, and 14 

 on the ventral valve (Figs. 18, 19). 



