418 



among all Mr. Cuming's dredgings at the Philippines, not a single 

 specimen has been found. 



The earliest known appearance of Ccecidce on our globe was in 

 the Eocene epoch ; three species having been found in the London 

 clay, belonging to the remarkable type Strebloceras, which is as it 

 were a permanently undeveloped Ccecum. In the Coralline Crag we 

 have three species which have since perished, while the recent B. 

 glabra appears to connect the ancient with the present condition of 

 our seas. No species have been found in the Eocene tertiaries of 

 the Paris basin (teste Deshayes, MS.) ; but one is said to have been 

 described by Grateloup from the middle epoch*. In Italy, species 

 have been found in the ? middle and later tertiaries ; one of which is 

 the C. trachea, now living in the same seas. 



Specific Characters. 



It is possible that all Ceecids are smooth, or nearly so, in their 

 nuclear and first tubular stage, assuming the characteristic sculp- 

 ture of the species either gradually or suddenly. The young shells 

 are much longer in proportion, and generally more bent, than the 

 adult ; the ribs and rings are either crowded or loose according to 

 circumstances ; and the mouth is often either sharp or thickened, 

 straight or slanting, according to age. To found species on shape 

 and sculpture, therefore, is more hazardous than in most shells ; 

 but there is one character which, with certain slight exceptions, is 

 found generally reliable. This is the shape (not the mere amount of 

 prominence) of the plug, which, instead of being of homogeneous 

 texture and simply flat or convex as in other septa-making and de- 

 collated univalves, always presents, in every successive stage of in- 

 crease, the same marked peculiarities of form and the same striae of 

 growth. This character having been generally overlooked by pre- 

 vious authors, fresh diagnoses have been written for all the known 

 species, in order that, by the use of terms in the same sense, the 

 forms might be more easily compared together. The lirce are 

 longitudinal riblets from apex to aperture, corresponding with spiral 

 sculpture in other shells ; the annuli form the concentric or trans- 

 verse sculpture. The septum is the plug covering the decollated 

 portion ; its mar go lateralis, the profile when the shell lies on its 

 side. The plug itself is either ungulate, when the whole surface 

 rises gradually towards the back like a horse's hoof; or mucronate, 

 when it tapers to a point ; or mamillate, when it is rounded, without 

 apex ; or it is intermediate between these forms. The groups de- 

 scribed under Ccecum can scarcely be regarded even as subgenera, so 

 very gradually do they pass one into the other ; but they are found 

 convenient, to avoid the frequent repetition of characters, and to aid 

 in the identification of species. 



* Teste Desh. in lit. But Dr. Gray, who has kindly consulted for me the Con- 

 chologie Fossile, 1840, is unable to find any allusion to the genus. 



