270 Mr. W. Clark on Scissurella crispata. 



eiderations ; and they can only be admitted to a provisional 

 position to await the rectifications that may be required by the 

 discovery of the animal architect. Conchological dispositions 

 must ever be the mere arena for surmise and doubtful classifi- 

 cation. 



Mr. Barrett has not stated his views on the systematic place 

 of Scissurella. Whether this has arisen from omission, doubt, 

 or from the idea of this species having already been properly 

 deposited, does not appear. My object is to supply some in- 

 formation on this point. 



Mr. Barrett's figure is that of a decided Trochidan animal : 

 this is evident from the ciliated tentacula and lateral vibracula, 

 which latter organs are peculiar to the Trochidce', the four 

 neck-lappets are composed of a lamina on each side, so deeply 

 indented as to cause each to appear as two distinct processes ; 

 these appendages are the invariable concomitants of the Trochi. 



Compare these points with the rough woodcut of the animal 

 of Trochus serpuloides of Montagu, published in the 'Annals,' 

 vol. viii. p. 45, N. S., and allowing a margin for specialty-varia- 

 tions, all the generic essentials of the Trochidan animal are incon- 

 testably apparent. 



The late Professor Forbes is one of the naturalists who has, in 

 his and Mr. Hanley's 'British Mollusca,' rightly placed this 

 species with the Trochidm ; but he acknowledges having adopted 

 that position from some notes of Sars ; he however speaks with 

 doubt, and hopes the animal will be better known. 



Mr. Barrett's discovery shows that the undoubted natural 

 position of Scissurella is with the subdiscoidal section of the 

 delicate minute Trochi; its congeners are the Trochus serpu- 

 loides, Mont., T. Cutleriana, mihi, and T. nitens of Philippi, 

 all the animals of which were first discovered by me, and de- 

 scribed as Trochidans. The operculum of these species is cir- 

 cular, with a subcentral nucleus and coarse spiral striae; that of 

 Mr. Barrett's Scissurella slightly difi'ers. They are all mere 

 films, and require a good lenticular power to distinguish accu- 

 rately their form and sculpture. 



The fissure in the centre of the outer margin of the shell of 

 the Scissurella originates in the animal specialty of a prolongation 

 of the points of the branchial organ to procure a more free 

 access of the water, and which by its presence, for a time, in- 

 terrupts at that particular point the testaceous exudation from 

 the mantle, and thus produces a short fissure, which in progress 

 of growth becomes obhterated in consequence of the branchial 

 impediments to the action of the mantle being removed to a new 

 portion of the shell, and in this manner a constant continuation 

 of the fissure is effected. But surelv this scission cannot be 



