Elements of conchology Prepared Elements of conchology / Prepared for the use of schools and colleges elementsofconcho00rusc Year: 1844 TROCHTJS. 45 28. The family of Trochoides are distinguished from other pectinibranch gasteropods without a , 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, a
Elements of conchology Prepared Elements of conchology / Prepared for the use of schools and colleges elementsofconcho00rusc Year: 1844 TROCHTJS. 45 28. The family of Trochoides are distinguished from other pectinibranch gasteropods without a , 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
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