316 Systematic Paleontology. 



with a single projection each side and sinus at tip, third lobe dilated, with 

 a small sinus each side and more obtuse and profound one at tip, posterior 

 lobe hardly larger than the lateral intermediate ones. Greatest diameter 

 of the transverse section 1.2 in., smaller diameter 0.7 in. ; length of the seg- 

 ment about 0.5 in." — Say, ] 820. 



" Shell attaining a large size, elongated and rather gradually tapering ; 

 section ovate, the antisiphonal side being more broadly rounded than the 

 opposite (or very rarely a little flattened ?) ; aperture of the same form 

 as the transverse section; extension of the lip on the siphonal side long, 

 tapering, and narrowly rounded at the end ; lateral sinuses of same deep 

 and about one-half to one-third the greater diameter of the shell; anti- 

 siphonal margin of the lip prominently rounded in outline; surface of 

 young and medium-sized specimens generally nearly smooth, while the 

 non-septate part of the adult shell is provided with broad, undefined, 

 obliquely-transverse ridges, or undulations, that arch parallel to the 

 obscure lines of growth, and become nearly or quite obsolete as they 

 approach the siphonal side, on which they are rarely represented by very 

 small, irregular ridges, scarcely distinct from the marks of growth. 



" Septa moderately closely arranged, or sometimes a little crowded ; 

 siphonal lobe nearly twice as wide as long, and provided with two large 

 terminal widely separated, more or less spreading branches, each of which 

 has sometimes three, and sometimes two, nearly equal, digitate branchlets 

 at the end, and two or three similar lateral ones on the outer side; first 

 lateral sinus about as wide as long, but narrower than the siphonal lobe, 

 and divided at the free end into two short, nearly equal branches, each of 

 which is again less deeply subdivided into about two to three or four 

 sinuous, spreading and digitate branchlets ; first lateral lobe oblong-ovate, 

 being longer and narrower than the siphonal lobe, and deeply divided at 

 its end into two very nearly equal branches, with each four to five spread- 

 ing and digitate subdivisions, in part generally so arranged as to give the 

 main branches a tripartite appearance at their extremities ; second lateral 

 sinus of nearly the same size as the first, and, excepting in unimportant 

 details, similarly branched and subdivided; second lateral lobe broader 

 and shorter than the first, and bearing two large, equal tripartite, sinuous, 



