141 



Tlie ProsobranchIata, the most highly organised as well as the most 

 numerous of the class, are nearly all marine, and protected by a spiral shell ; 

 they derive their name from the position of their gills in front of the heart : the 

 comh-Ukt sti-ucture of these organs originated Curier's appellation Pectini- 

 branchiata for a group nearly co-extensive with this order. The aperture of 

 some of their shells is notched or produced into a canal, indicating the passage 

 of a tubular prolongation of the mantle, by means of which as by a siphon 

 (hence the name of the section Siphonostomata) water is conveyed to the gills, 

 without necessitating the egress of the animal itself from its shell : Examples of 

 this organization are the well-known whelks, cones, and cowiues. 



These siphonated Glasteropods are mostly carnivorous ; they have not been 

 found in rocks earlier than the Liassic series ; their numbers vastly increase in 

 the Tertiary formations, and they still abound on every sea shore. 



The noi-mal Silurian Gasteropods belong to the Holostomatous, or whole- 

 motUhed section of the Prosobranchiate order. These vegetarian sea-snails have 

 left their petrified shells in all fossUiferous rocks from the LlandeUo flags 

 upwards ; only a few, however, of the families are certainly known to have 

 been represented in Silurian seas. Murchison, in the new edition of Siluria, 

 gives 97 species, distributed among 20 genera : of these last the Turhinidce claim 

 the most, although, according to Owen (Paleontology, p. 92), no true Turbo is 

 known from strata before the Cretaceous ; to this family belong the Euomphali 

 (very common at Dormington Wood), of one species of which we give a drawing 

 from a magnificent specimen belonging to R. Johnson, Esq., the Cyclonemas, 

 Eunemas, and others whose generic names are still unsettled. Loxonema belongs 

 to the Pyramidellidcc, and Acroculia to the Calyptrmidm, or bonnet-limpets. A 

 common trochiform genus of the family Haliotidce, having its whorls marked by 

 a peculiar band, usually terminating in a deep sUt at the aperture, is called 

 Pleurotomaria ; and a smaller and slenderer sort, well represented in all but 

 the Wenlock series, has received the name Murchisonia, from the veteran 

 Silurian geologist. The lanthinidce include the Lower Silurian Holopea and 

 Baphistoma. To the TurriteUidce belong the Holopellas, bearing some resemblance 

 to the Loxonema of our sketch. Natica, Patella, and Chiton are all doubtful. 



"We have lastly to describe the curions aberrant order Nucleobeanchiata, 

 so called because the breathing organs form a nucleus on the back. Some 

 naturalists have formed of them a separate class, to which they have given the 

 name Heteropoda, from the peculiar alienation of the foot from the normal 

 Gasteropodous type. McCoy has differently arranged the Silurian genera 

 usually placed with this order, setting the Maclureas with the Prosobranchiate 

 family Trochida (Turhinidce), and raising the Bellerophons to the class 

 Cephalopoda ; we have, however, followed Owen, Woodward, and Carpenter in 

 placing them with this order at the end of the Gasteropoda, not because of any 

 inferiority of organization, but to mark their divergence from the type of the 



