254 SYSTEMATIC PALEOKTOLOGY 



the male measured 40 mm. in maximum diameter and the female 45 mm. 

 The difference in form is even greater than these measurements would 

 impl}'. The deep-water specimens are thin but growing to a verj^ large 

 size. This species is especially subject to decortication in the fossil state, 

 and when so mutilated is difficult to recognize. Perfect adult specimens 

 can usually be identified liy their globose form, rounded but not turreted 

 whorls, small ribless umbilicus and feeble callus. The young resemble 

 L. interna, but want the umbilical rib, though it is sometimes quite diffi- 

 cult to separate immature specimens." 



Dr. Dall also recognized interna Say and perspectiva Eogers as distinct 

 species occurring in the Maryland Miocene. In regard to the former he 

 says : " Limatia interna may be discriminated by its low spire, its full 

 and rounded whorls, and by the characters of the umbilicus, which shows 

 a marked sulcus ascending spirally below a thickened, obscure rib, which 

 are respectively indicated in mature and perfect specimens by an emargi- 

 nation and a callus on the pillar-lip. It has six or eight whorls, and 

 attains a breadth of 2G and a height of 28 mm." 



He defines perspectiva as follows : 



" This is N. heros Conrad, ex parte, non Say, and N. hemicrijpta Con- 

 rad, ex parte, not of Gabb. The species is much the same shape as L. 

 triserialis Say, of the recent fauna, but is larger, heavier, and with a dif- 

 ferent umbilicus. It may be recognized by its smoothly arched spire, in 

 Avhich the rotundity of the whorls is not marked and the suture is smoothly 

 appressed, as in a male Neverita duplicata of the elevated variety; by 

 its umbilicus, which is wide below and obscurely spirally striate, with 

 near the top of the umbilical wall a sharp, narrow spiral rib, which ter- 

 minates between two obscure notches on the columellar callus. It is a 

 narrower, heavier and smaller shell than the average adult L. heros, 

 though decorticated specimens such as abound in the marls are difficult 

 to recognize." 



After a very careful study of large collections from all the Maryland 

 horizons and localities it has .been impossible to separate the material 

 into the species recognized and re-defined by Dr. Dall, and it appears best 

 for the present at least to refer all the fossil forms to L. heros. 



Height, 65 mm. ; diameter, 60 mm. 



