Family C^CIDjE. 



Shell tubular, with a spiral plane nucleus which is caducous 

 or persistent, then becoming cylindrical, curved, terminating in 

 a simple, circular aperture, the posterior portion of the tube 

 usually divided by one or more septa (PL 66, figs. 47-51). 

 Operculum horny, multispiral, margin sometimes fimbriated. 

 There are usually three stages of growth in the shell of Ciecum : 

 first, the spiral or nuclear, soon lost by truncation, the end of 

 the remaining tube closed by a septum; second, the adolescent 

 stage, a curved tube, also lost subsequently' ; and, third, the 

 adult tube, of similar curved shape, and again closed behind by 

 a septum (PL 66, fig. 39). In Strebloceras the three stages of 

 growth are persistent and the septa consequently absent. 



Animal with a long flat rostrum ; tentacles cylindrical, with 

 sessile eyes at their outer base ; mantle thick, fleshy, circular, 

 closely embracing the neck ; a single branchial plume ; foot 

 short, narrow, truncated in front, attenuated and obtuse behind. 

 Dentition 2-1-2 ? ^ 



These minute mollusks have some points of resemblance with 

 the Vermetidae, but are alwaj's free and the foot is without the 

 anterior tentacular projections which distinguish that family. 

 The animal is not at all sh}^, and crawls with considerable vivacity. 

 The Caecidae, on account of their minuteness, have been neg- 

 lected by collectors and students. No complete illustrnted 

 monograph of the family exists. P. P. Carpenter was the first 

 naturalist to study them, and we are indebted to him for some 

 of the subdivisions of the famil}- at present recognized, as well 

 as for others which must be relegated to the synonymy. Unfor- 

 tunately the obvious distinctions of sculpture, often so charac- 

 teristic of species and higher groups, have not the S3'stematic 

 importance here which Carpenter supposed them to have ; the 

 Marquis de Folin, the latest monographer of the group, having 

 shown that the same species will sometimes varj' from a smooth 

 to a ribbed surface. For this reason, and also because most of 

 the species have not been figured, our knowledge of them 



(213) 



