296 TRANSACTIONS OF THE WAGNER FREE 



whorl extends as a gently arched tube. After a time this tube enlarges suffi- 

 ciently to contain all the soft parts, and they are moved forward into the newer 

 portion, which is then separated from the shell of the first stage by the forma- 

 tion of a " plug " or "septum " which entirely separates the cavity into two 

 parts, shortly after which the vacated portion drops off and is lost, though in 

 rare instances it is retained abnormally. The shell of the second stage has 

 only traces of the adult sculpture, annulate species may be nearly smooth 

 when in the second stage, etc., and when the second stage is complete the be- 

 ginning of the third stage, which continues straight with a dorsal arch, is 

 usually marked by a sudden and considerable increase in the diameter of the 

 tube. The animal now assumes its adult characteristics, grows steadily until 

 the shell of the third stage contains the whole of the soft parts, a new plug is 

 then formed and the empty secondary shell is dropped in its turn. The creat- 

 ure has a multi-spiral horny operculum, and the external form of the plug, 

 rounded, pointed or with a spurlike projecting process, is characteristic for the 

 species. The external surface of the shell may be smooth, longitudinally 

 sulcate, annulate or reticulated, and is sometimes even spinose. The recent 

 shell is usually brownish, white, or mottled yellow and white. 



The genus Strebloceras differs from Ccecitm by the asymmetry of its 

 planorboid nucleus and the fact that instead of being dropped it is retained 

 through life, no septa being formed. The group is chiefly fossil and indi- 

 cates the formative period of the Ccecidm, when the habit of dropping the 

 early stages had not become fixed. It is questionable whether the only re- 

 cent species is not founded on an abnormal atavistic individual of the typical 

 genus. 



The genus Parastropliia has a small, loosely coiled nucleus of a single 

 whorl ; where this is dropped the posterior end is sealed by a secretion of 

 shelly matter, no plug is formed and the shell continues to grow as a slightly 

 sigmoid, rapidly enlarging cone. In this type there are only these two stages. 

 Watsonia is quite similar in a general way, but shorter, more inflated and with 

 a singularly oblique orifice. 



Meioceras begins like Cacmn, but the second stage is shaped like a cow's 

 horn, rapidly enlarging ; the third stage is still more different, short, mostly 

 smooth and equatorially tumid and but little arched. These stages are remark- 

 ably different, and it is not very surprising that O. Meyer described the second 

 stage of this genus as a Pteropod, under the name of Bovicormi, although it 

 had been figured and described by De Folin eleven years previously. 



In studying the American species of this difficult group, I have had the 

 kind assistance of the Marquis Leopold de Folin, who has examined speci- 

 mens of all the species here mentioned, aided by his unrivalled collection of 

 specimens of this genus. 



