INVERTKBKATK PAL.EONTOLOC.Y. 75 



transversely oval-oblong, ventricose or rather compressed; beaks anterior; 

 surface covered with an epidermis, and ornamented, wholly or in part, by 

 radiating, sometimes divaricately-arranged, striai; hinge nearly or quite eden- 

 tulous, but generally crenate; ligament internal; muscular impressions two, 

 faintly marked, unequal; pallia! impression obscure, entire. 



It seems to me somewhat doubtful whether Modiolaria ought not rather 

 to be regarded as a distinct genus, than to be included as a subgenus under 

 Crenella, as is done by H. and A. Adams. The shell of this type has much 

 more the form of Modiolu, but differs in its markings, while the animal is much 

 more like that of Crenella. Including Modlolaria as a subgenus under Cre- 

 nella, the two sections may be separately defined as follows: 



1. crenella, Brown (typical). 



Shell short, suborbicular, or nearly vertically ovate; surface cov- 

 ered by radiating striae. — (Type as stated above.) 



2. modiolaria, Beck. 



Shell transversely oblong or oval, with beaks depressed, and sur- 

 face-striae obsolete on the central parts of the valves. — M.discors, Linn. 



Stalagmium, Conrad (— Myoparo, Lea), is sometimes separated, sub- 

 generically or otherwise, from Crenella, but it seems to agree so nearly with 

 the same as scarcely to deserve to be placed as a separate section. It is 

 probable that Nuculocardia, d'Orbigny, as typified by N. divaricata, d'Orbigny, 

 should also be included in this genus. 



The genus Crenella is, I believe, not certainly known from older forma- 

 tions than the Cretaceous. It is also represented in the Tertiary, and in the 

 seas of the present epoch, in which latter it probably attains its greatest 

 development — that is, including the section Modiolaria. Both sections, how- 

 ever, occur among the living species, while the older fossil species seem all to 

 belong to the typical section. The living species have a wide distribution, and 

 show a disposition to conceal themselves among sea-weeds, corals, &c, in a 



kind of nest. 



Crenel] a e I e g a ii i u I a , m & H. 



Plate 28, tigs. 6, «, b, c. 

 Crenella ehganiula, Meek and Hayden (1861), Proceed. Acad. Nat. Sci. Philad., XIII, 441. 



Shell nearly vertically ovate-cordate, very thin, ventricose; postero-basal 

 and basal margins rounded; dorsal border sloping abruptly behind, with a 



