INSTITUTE OF SCIENCE, PHILADELPHIA. 341 



Rissoa (Onoba) callistrophia n. s. 

 Plate 21, figure 14 a. 



Pliocene marls of the Caloosahatchie and Shell Creek, Florida, Dall and 

 VVillcox. 



Shell elongate, slender, loosely and somewhat irregularly coiled, with 

 seven rounded whorls ; nuclear whorl very minute, smooth, the remainder 

 with close-set, somewhat alternated, fine revolving flat threads, covering the 

 whole surface ; whorls somewhat shouldered, spire subacute; base rounded, 

 aperture small, angular behind, rounded in front, continuous, though thin, 

 over the body ; pillar-lip reflected, forming a minute chink, but not a per- 

 foration ; margin thin, obtuse. Alt. specimen A, 4.6; B, 4.75 ; max diam. A, 

 19; B, 1.8 mm. 



The loose coiling of this shell gives rise to obvious differences of appear- 

 ance. It is related to R. {Onobd) aciileus Gould, but is larger, less pupiform, 

 with less regular striation and a proportionately much smaller mouth. 



Rissoa (Onoba) miorocharia n. s. 

 Plate 20, figure S a. 



Pliocene of the Caloosahatchie, Dall. 



Shell very minute, with four rounded whorls ; nucleus inflated, polished, 

 remaining whorls smooth or marked only by faint incremental lines, gently 

 rounded; suture distinct ; base produced, aperture subovate, angular behind; 

 the margin thin, continuous, slightly reflected on the pillar, making an almost 

 invisible chink. Alt. of shell 1.12; max. diam. 0.6 mm. 



This is one of the most minute species known. It has much the form of 

 R. aculeus, but is without spiral sculpture ; the inflated nucleus at first sug- 

 gests that the shell is immature, but the other characters indicate maturity. 

 At any rate, it is not the young of any species now known from the Pliocene 

 marls. 



To conclude our observations on this genus, I may remark that Gabb re- 

 ports Rissoa Auberiana Orb., from the Pliocene of Costa Rica ; the Claibornian 

 Pasithea sulcata of Lea is a Rissoid with spiral sculpture recalling Onoba; 

 Pasithea cancellata H. C. Lea is a young Tuba, as the type-specimen shows, 

 though De Gregorio has referred it to Rissoa ; the Littorina fervens of the 

 last-mentioned author seems to me to belong rather in the genus Rissoa ; it 

 comes from the Claibornian, from which De Gregorio has also described a 

 Rissoa [Ahmnid) ziga. Rissoa {Ci^tgula) mimita Gould and R. obsoleta S. 

 Wood are reported by Dawson from the Post-Pliocene of Montreal, Canada. 



Genus RISSOINA Orbigny. 



So far, this genus has been hardly known from American Tertiaries. R. 

 plicato-varicosa Heilprin, of the Claibornian, is an Alaba, and the only other 



