MARYLAND GEOLOGICAL SURVEY 623 



Dimensions. Altitude 11.8 mm., latitude 21.8 mm., diameter of 

 double valves 11.7 mm. 



Lithophaga twitchelli suggests, at first, a giant L. ripleyana Gabb. How- 

 ever, L. twitchelli is not only a third as large again as Gabb's species, but, 

 furthermore, the valves are very much more inflated, particularly along 

 the diagonal; the umbones feebler and the medial depression more pro- 

 nounced. Then, too, the shell is much heavier, apparently, and there is 

 no evidence of the former presence of an encrustation. 



This species is named for its collector. Dr. Mayville W. Twitchell, 

 Assistant State Geologist of New Jersey. 



The form is described from a cast of the double valves of a single indi- 

 vidual to which a considerable amount of shell substance still adheres, 

 although the external surface has been entirely decorticated. 



Occurrence. MONMOUTH FORMATION. Eailroad cut west of Seat 

 Pleasant, Prince George's County. 



Collection. Maryland Geological Survey. 



Genus CRENELLA Brown 



[111. Conch. Gr. Brit., 1827, pi. xxxi, figs. 12-14; 2d ed., 1844, p. 75, pi. xxiii, figs. 

 12-14. Not Crenella Sowerby] 



Type. Mytilus decussatus Laskey. 



" Shell oblong-oval, equilateral, ventricose ; beaks obtuse, slightly turned 

 to one side; hinge destitute of teeth but with a flattened, horizontal, 

 slightly crenated plate on one side of the hinge in each valve ; right valve 

 with a triangular, horizontal, projecting, reflexed plate, and the left one 

 witli an oblique plate, both of which are a little crenated externally ."- 

 Brown, 1844. 



" This interesting little group extends through the Tertiary and, owing 

 to the little study given to its characters, has received many names. The 

 shell is usually convex and ovoid, with more or less incurved beaks, a 

 nacreous inner layer, thin epidermis which adheres closely to the shell, 

 and a fine radial, often crossed by a concentric striation. In young shells 

 the provinculum is exceptionally well developed, sometimes recalling the 



Etymology: Diminutive of crena, notch. 



