INSTITUTE OF SCIENCE, PHILADELPHIA. 337 



the umbilicus of the same width, but is a decidedly narrower shell." [H. A. 



Hydrobia umbilicata Pilsbry, n. s, 



Plate 2t, figure 9 a. 



' Pliocene marls of the Caloosahatchie River, Florida, Dall and Willcox. 



" Shell umbilicated and of ovate-conic form ; apex subacute ; sutures 

 well impressed; whorls five [or six], moderately convex, the body-whorl, 

 especially in the young, being more or less distinctly angulated at the periph- 

 ery, and convex beneath, especially around the umbilical perforation ; aperture 

 ovate, not oblique, its length contained 2'-^ times in tire length of the shell. 

 Alt. Ti.6; diam. 2.2 mm. 



" This species is not closely allied to any American forms except Hydro- 

 bia amnicoloides, and in a less degree to H. miimta Totten. The umbilical 

 perforation and the more or less obvious peripheral keel are its most striking 

 features." [H. A. P.] Since the above was written some larger specimens 

 have come to light, which have the keel less marked, the umbilicus a little 

 smaller, and one more whorl (six in all). They measure in alt. 6.0 ; max. 

 diam. 2.5 ; alt. of aperture 2.25 mm. One of these has been figured to illus- 

 trate the species. 



Hydrobia mobiliana Dall, n. s. 



Grand Gulf Miocene beds of Alabama, from an artesian well near Mobile, 

 at the depth of 735 feet ; Lewis Woolman. 



Whorls about six, ovate-conic, with a rather blunt apex ; early whorls 

 rounded, later ones flattened, sloping suddenly from the flattish part to the 

 very distinct suture ; body-whorl rounded below, with a very minute umbili- 

 cal perforation; surface polished, without obvious spiral striae; aperture 

 rounded in front, angular behind, continuous, with a rather obtuse margin, 

 the substance of the shell thick and solid. Alt. of shell about 2.5 ; max. 

 diam. 1.3 mm. 



The specimens, having been subjected to the chisel used in boring the 

 artesian well, are reduced to fragments, from which the above description has 

 been obtained. The young specimens have a decided peripheral angle which 

 is lost in the adult. The flattening of the side of the later whorls, and the 

 manner in which they fall into the suture, gives the latter a channelled appear- 

 ance by which the .species can easily be recognized. The very great rarity of 

 moUuscan remains in the Grand Gulf beds gives them an exceptional im- 

 portance which renders it desirable to name even such imperfect material. 

 With them were found young specimens of Gnathodon Jolinsoni Dall, n. s., 

 which will be figured later in this paper,* and minute fragments of a Corbiada. 

 The Gnathodon, together with Mactra lateralis Say, var., were collected in the 



* See plate 22, figure IS. 



