536 



MOLLUSCA 



PHYLÜM VI 



reproductive organs and nervous System agree with tlie correspondiiig organs of the 

 Ctenobranchiates ; tlie radula resembles that of tlie Taenioglossa. Tliey differ con- 

 siderably, however, from the Prosobrancliiates, since the foot is modified into a sort of 

 vertical fin, and imparts to them a peciüiar appearaiice. They 

 rise usually toward evening in great swarms to the surface of 

 the ocean, where they hover about with a very rapid motion, 

 swimming in an inverted position, with the dorsal side down, 

 and the foot uppermost. They are exceedingly delicate, often 

 transparent organisms, The body may be either entirely 

 naked or provided with a very thin, light shell. 



Two Recent genera have been found also in early Tertiary 

 deposits. Of these Garinaria Lamarck, has a keeled, cap-shaped, 

 glassy shell ; while in Atlanta Lesson (Fig. 909), the delicate 

 shell is coiled spirally in a single plane, and the apertiire is provided with a slit. 

 Owing to a similarity in coiling of Atlanta and Oxygyrus to that of the Paleozoic 

 Bellerophontidae, a relationship between the two has been suggested. The latter 

 forms are distinguished by their heavier, thicker shells, but are very pi'obably related 

 to Emarginula and its allies. 



Fio. 909. 



Atlanta peronii Lesueur. 

 Recent ; Atlantic Ocean. 



Suborder B. PLATYPODA Lamarck. 



Fig. 910. 



Superfamily 1. GYMNOGLOSSA Gray. 



Mostly Jiolostomate forms, in which the radula is usually unarmed through degenera- 

 tion. The smaller forms frequently parasitic or commensal. 



Family 1. Eulimidae Fischer. 



Small, polished, elongate-conic shells, with ovate aper- 

 tures ; the axis often ddstorted, protoconch dextral. Trias 

 to Recent. 



Eulima Risso {Melanella Bowdich) (Fig. 910). Tur- 

 reted, smooth, lustrous, without umbilicus. Trias to 

 Recent. 



Niso Risso (Fig. 911). Like the last, but with deep 

 umbilicus reaching to the apex. Trias to Recent. 



A, Eulima suh 

 ulata Don. Plio 



Family 2. Pyramidellidae Gray. cene ; Coroncina 



Tuscany. B, E. 



Shell turreted to elongate-oval. Aperture oval, an- poiita (Linn.), 

 teriorly rounded, or angular ; outer lip sharp. Operculum leis, Moravin. 

 horny, spiral. Marine. Cambrian to Recent. 



The protoconch consists of several whorls, and in Paleozoic and Mesozoic forms is 

 coiled in the same direction as the remainder of the shell. But in the younger and 

 more typical genera it is heterostrophic, distinctly separated from the rest of the 

 shell, and often Stands at an angle with the adult spire. It is questionable whether 

 forms older than the Cenozoic can be retained in this family ; Fischer places most of 

 thent in a new family, entitled Pseudomelaniidae. 



Macrocheilus Phil. {Macrochilina Bayle ; Strohaeus de Kon.) (Fig. 9 1 2). Elongate- 

 oval, without umbilicus, smooth or with slightly curved growth-lines. Spire acuminate, 

 only moderately high ; last whorl large. Aperture angular posteriorly, sometimes 

 also in front. Inner lip with weak anterior folds. Silurian to Trias. 



(?) Ptychostoma Laube ; (?) Undularia Koken. Trias. 



Loxonema Phil. Turreted, whorls arched, with S-shaped growth-lines. Sutures 



Fig. 911. 



Niso eburnea 

 Risso. Plio- 

 cene ; Monte 

 Mario, near 

 Rome. 



