330 LOWER IXVERTEBRATES. 



papalis, which lias each whorl of the sph-e crowned with knobs, and tlie small colored 

 spots of the shell much more irregular in shape and distribution than in M. episcopalis. 

 The genus Mitra contains over two hundred species, a large proportion of which come 

 from the Philippines and the neighboring seas. Almost all of the genus are tropical 

 and semitropical ; several being found in the West Indies ; but an exception to this 

 distribution is found in M. gronlandica, as its name indicates, an Arctic form, which, 

 on account of peculiarities of its lingual dentition, has been se23arated as a sub-genus 

 Volutomitra. 



The family Mueicid^, as at present limited, embraces a heterogeneous assemblage 

 of forms. Several attempts have been made to divide it without doing violence to 

 the affinities of one or more genera, but no scheme has as yet received universal accep- 

 tance. If we base the division on the lingual dentition, it does not agree with char- 

 acters derived from tne animal and from the shell ; if on the anatomy of the animals, 

 still other features are not in accord, etc. With this uncertainty it is best, at least in a 

 popular work, to leave the classification in its present condition, and to define the 

 family as embracing a group of molluscs, in which the foot is broad and of moderate 

 length, the siphon long, the eyes at the base of the tentacles, while the characters derived 

 from the shell are the presence of a long or short, straight, anterior canal, and an oval 

 operculum with the nucleus at the smaller end. Necessarily where so much con- 

 fusion exists there will be an inequality in the relative rank of certain of the included 

 types, and in the following remarks some genera named will possibly not be worthy of 

 generic rank, while others, on the other hand, may deserve to be regarded as really 

 of the grade of sub-families. The same trouble also occurs with the next family, the 

 BuccinidsE. 



The first sub-family, the Muriciure, is well marked by characters derived from the 

 shell. The growth is apparently marked by periods of rest, and at each of these the 

 aperture is thickened and marked by ornamentations of various kinds. Then the shell 

 grows again, and shortly another period of rest ensues, when the nodes, spines, or 

 thickenings (varices they are called) of the mouth are repeated. These interruptions 

 occur at varying intervals in different species, and are of some use in defining 



generic limits. A similar process occurs in some 

 other families. 



The typical genus is Murex, in which the canal 

 is long and straight, the aperture round, and the 

 shell is interrupted by varices and spines at least 

 three times in the course of the growth of the whorl. 

 In the colder waters the colors are subdued, and the 

 shell does not acquire that fantastic form that is fre- 

 quent in the tropical species. The species are among 

 the most rapacious of molluscs, boring through the 

 shells of other si)ecies in the same way as does the 

 JVatica, to be described on a subsequent page. In 

 Europe, Mure.r erinaceus does great damage to the 

 oyster beds. Allied species {31. brandaris and 31. 

 „ ,,„ ,^ ,. «rMn.cwfe<s) were employed by the ancient inhabitants 



Fig. 412. — Murex endiva. ' r j j 



of Syria and Greece in the preparation of the cele- 

 brated Tyrian purple. Of the two hundred and odd species of this genus, we need 

 only mention, in addition to those just referred to, the 3furex tenuispina, in which the 



