EXTERNAL FORM OF SHELLS. 417 



tinct species, under the name of Buccinum acuminatum. 

 Sometimes, after the occurrence of such an accident to a 

 spiral shell, the form or sculpture of the whorls is entirely 

 altered : they often become ventricose and smooth, instead 

 of remaining thick and ribbed, as in the common state of 

 the shell ; and this change of form has even occasioned shells 

 under such circumstances to be regarded as distinct species. 

 As an instance of this I may refer to Cingula alba of Dr. 

 Fleming, founded on specimens of Turbo parvus of Montagu, 

 which had been injured in their growth. In Mr. Turner's 

 collection there is a specimen of Terebra maculata, which 

 had sustained an injury when about an inch and a half in 

 length : the whorls beyond this injury are rounded and ele- 

 vated near the suture, and are destitute of the posterior 

 groove. In this specimen the colouring also is altered, for 

 instead of being spotted, the irregular volutions are marked 

 only with two narrow posterior spiral bands. 



A distortion or change of form, caused by a fracture or 

 other accident, is sometimes overcome, as the animal in- 

 creases in size and recovers its strength. Thus a specimen 

 of Strombus bituberculatus in my collection, which had met 

 with an accident in its young state, has the five upper whorls 

 sharply keeled and nodulous, as in the ordinary state of the 

 shell, but the volutions formed after the accident turn more 

 obliquely down the axis, and thus their anterior part be- 

 comes more exposed, leaving a deep narrow groove on the 

 suture. The first half-whorl that succeeds the fracture is 

 rounded and distinctly tubercular ; after which the shell con- 

 tinues rounded, but quite smooth for a whorl and a half ; 

 when it again becomes slightly tubercular, and at length 

 keeled and tubercular, the last whorl exactly resembling the 

 last whorl of the normal shell. The collection of Mr. Lin- 

 come, of Highbury, contains a specimen of Cassis rufa in a 

 very similar state. 



The shape of attached shells depends greatly on the form 

 of the bodies to which they are applied ; and this is a cir- 

 cumstance that has been generally overlooked by concholo- 

 gists. It strongly affects most shells that are immediately 

 and permanently attached, such as the Chamae and Ostreae, 

 which completely assume the form of the substances on 

 which they grow.* Thus the Chamae and Spondyli attached 



* Of the genus Chama, Mr. Broderip says : " These shells appear to 

 be subject to every change of shape, and often of colour, that the accidents 

 of their position may bring upon them. Their shape is usually determined 

 by the body to which they are fixed ; the development of the foliated 

 laminae which form their general characteristic is affected by their situation ; 



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