TROCHTJS. 45 



28. The family of Trochoides are distinguished from other 

 pectinibranch gasteropods without a sy.phon, by the form of their 

 shelly which is spiral, with an entire aperture, and by the exist- 

 ence of an operculum, or some organ m the place of it. This 

 family is divided into three tribes, which are distinguished by 

 the form of the opening of the shell ; namely, Troclms, in which 

 ithis opening, angular at its external edge, is nearly quadrangular, 

 and, as regards the axis of the shell, lies in an oblique plane 

 {Figs. 85 and 36) ; the tribe Turho, in which the mouth of the 

 shell is perfectly round {Figs. 43, 45 and 46), and the tribe of 

 Hemicyclostoma, in which this opening is more or less in the 

 form of a crescent, as in the Helices {Figs. 26 and 27). 



29. The Troghus~so called from the general form of the 

 shell, that of a top; its spire is more or less raised, and terminates 

 in a point, and the circumference is trenchant or carinate : ani- 

 mals of this tribe are mollnsks with a short foot, the mantle or- 

 dinarily furnished on the edge with two or three pairs of filiform 

 appendages, the two eyes being placed on a swelling at the base 

 of the tentacles, the mouth armed with a sort of spirally rolled 

 tongue, and the anus opens on the right side of the respiratory 

 cavity, which encloses two unequal branchiae. The operculum 

 is horny, and marked by numerous spiral turns. The tribe of 

 Trochus is divided into several little groups, according to the ab- 

 sence or presence of an umbilicus, and some other not very im- 

 portant characters. Figure 35 gives a view of the base of the 

 Trochus niloticus^ showing the umbilicus or hole in the buse of 



u 



Fig. 35. TROCHUS NiLOTicus. Fig. 36. 



the columella (w), while Figure 36 is another view of the same 

 shell. The Solarium is a trochus in which the shell is conical in 



28. What are the peculiarities of the family of Trochoides ? (pronounced 

 tro-Jcoi-des.) Into how many tribes is this family divided ? 



29. What are the characters of the tribe of Trochus ? (pronounced 

 tro-kus.) 



