3Ionograph of the Gasteropoda. 30") 



first hatched, with a closed opercuhim. Instead of creeping while 

 young, they swim with a pair of ciliated fins springing from the sides 

 of the head. Their development may be profitably studied in the 

 Melania and Puliidina of the Ohio river. 



Cuvier divided the branchifera into the following orders : 1. Pec- 

 tinibranchiata ; 2. Scutihranchiata ; 3. Cyclobrauchiata ; 4. TubuUbran- 

 chiata ; 5. Tectibranchiata ; 6. Inferobranchiata; 7. Nudibranchiata. 

 Later authors have united the first four of the above orders into one 

 order, calling it the Prosobranehiata, and the last three into another 

 order, calling it the Oputhobrancldata. The OpisthobrancJiiata are all 

 hermaphrodites, and very few of them are inclosed in shells ; they are 

 not known in the Cincinnati Group. 



The Gasteropoda of the Cincinnati Group, therefore, belong to the 

 NudeobrancJiiata and the Prosobranehiata. 



The Nudeobraneliiata (Blainville) are so called because the respira- 

 tory and digestive organs form a nucleus on the posterior part of the 

 back. They are all inhabitants of the ocean, where they swim about 

 raj)idly, instead of creeping on the bed. This order is divided into two 

 families: 1. Atlantidce; 2. i^iroZfVte, and some authors include a third 

 small family, Sagittidce. The Atlantido; have a discoidal shell, large 

 enough to contain the whole animal when contracted ; and the gills 

 are contained in a regular branchial cavity. Sometimes they possess 

 a delicate operculum. This family has been subdivided into various 

 families, as Bellerophontidce, etc. It includes the following genera of 

 this monograph, viz.: Bellerophon, Bucania, Cyrtolites, Microceras. The 

 Firolidce are either entirely naked, or furnished with a small, conical 

 keeled shell, which incloses the intestinal nucleus, and which very 

 much resembles the Carinaropsis, the latter genus may not, however, 

 belong to this family. 



The Prosobranehiata (M. Edwards) have a spiral shell, into which 

 they can retract themselv^es at pleasure ; the mantle forms an arched 

 chamber over the back of the head, in which the branchiaj are- 

 situated, together with the orifices of the alimentary and generative- 

 organs. The branchiiB are pectinated or plume-like, and situated iib 

 advance of the heart. The sexes are distinct and nearly all the species 

 are marine. This order is divided into more than twenty fixmilies, 

 and these again are divided into sub-families. The family Haliotidir 

 is characterized as follows : 



Shell varying in form from elongate conic to depressed and ear- 

 shaped ; aperture wide, indented by the last whorl ; nacreous, and 

 with one exception, having either a deep slit or row of foramina in the 

 thin, simjile, outer lip, leaving a band which extends back along the 



