BUI<I.KTIN 31 100 



that the St. Maurice representatives were early derived. In the 

 Jackson we find, strangely enough, as in the case of Venericardia 

 rotunda, species or varieties, according to the taste of the investi- 

 gator, more nearly related to the St. Maurice than to the Clai- 

 borne representatives. Such being the case it would seem that 

 the whole protexta stock could well be referred to one and the 

 same species while a number of names could be used to designate 

 forms that have a somewhat important stratigraphic or geograph- 

 ic significance, — constantly bearing in mind, however, that these 

 forms occasionally imperceptibly intergrade. But for the pres- 

 ent, in order not to disturb the old nomenclature any more than 

 is absolutely necessary, we will treat of the protexta stock under 

 three "species", corresponding with the forms found in the St. 

 Maurice, Claiborne and Jackson stages respectively and then in- 

 dicate such varietal forms as seem worthy of consideration. The 

 general scheme of relationships may be represented thus : 



Jackson flexurtis 



var. fiexurus, producius, postclarkensis 



Claiborne protextus 



St. Maurice var. hidoviciamts clarkensis ferrocarolinus 

 clarkensis 



Crassatellites clarkensis Dall, PI. 34. Figs. 5-10. 



C. clarkensis Dall, Trans. Wag., Ill, 1900, pi. 36, fig. 20, 21, 24, 25. 



C. clarkensis Dall, idetn, 1903, p. 1470. 



DaWs original description. — Shell solid, elongated, moderately thick, 

 inequilateral, the anterior end shorter, nearly equivalve ; anterior end 

 rounded, posterior end obliquely descending, pointed and subrostrate, with 

 a marked carina from the umbo to the posterior angle ; lunule and escutch- 

 eon subequal, lanceolate impressed ; surface smooth except for incremental 

 rugse and microscopic radial striation ; nepionic shell small with about six 

 sharp, low, concentric ridges which cease abruptly in less than three milli- 

 meters from the apex ; hinge well developed ; scars of ligament and resil- 

 ium of moderate size; third right (posterior) cardinal nearly intact 

 and distinct ; anterior and basal margin sharply, finely crenate, the sub- 

 surface layer of the shell weathering out with distinct ribs corresponding to 

 crenations. Lon. 42.5, alt. 26.5, diam. 19.0 mm. 



This resembles C. protextus, from which it is easily separable by its 

 more pointed rostrum, sinuate below, and the very small space occupied by 



