260 PROCEEDINGS OF THE MALACOLOGICAL SOCIETY. 



External sculpture in the Gastropods is verj^ variable, and the shell 

 may be furnished within the mouth either with ridges or tubercles ; 

 the columella being often provided with accessory plates or folds : the 

 wonderful armature of the Helicoids, Cylindrellids, etc., at once comes 

 to mind. Shells such as Crepidula and Anomia will form ridges, 

 tubercles, etc., to match their site. While in many cases the internal 

 armature is specifically constant, in others it shows considerable 

 variation, such as the presence or absence of teeth in the Pupidae. 

 The crenulations on the margin in Astarte, again, have been recently ' 

 alleged to be sex characters, those with a crenulated margin being 

 said to be female, those with a smooth edge male ; and the point is 

 one of considerable importance since these characters have been used 

 for the separation of species. Dr. Dall states that in Alaska Littorina, 

 where exposed to the full power of the surf, becomes shorter in the 

 spire, the ridges become rounded nodules, and the axis of the shell 

 broad and thick ; an illustration of the principle that sculjjture is 

 largely due to the surroundings. 



Our common Purpura lapillus, again, varies greatly in sculpture, 

 some showing well-marked foliations iu addition to the usual spirals, 

 and it may be remarked that those in which the foliations were most 

 noteworthy come from an old oyster bed in Rhoscollyn Bay, where lime 

 would be abundant. 



The variation of the ribbing in Cardium has been noticed ' by Baker, 

 and, on comparing three species, he found that they differed in 

 the amount of variation, C. magnificum being the least variable, 

 C. muricatuni the most variable, while another form was intermediate. 

 Peden also has been carefully studied by Davenport,^ who found that 

 the same species was more variable in some places than in others : he 

 also states that the right valve has half a groove more than the left or 

 upper one, and that the variability of the right valve is less than that 

 of the left. 



The Grafin von Linden "• found on examination that the primary stage 

 in sculpture was, in the forms seen, usually a thickening of the lines 

 of growth to form transverse ridges ; then, firstly, transverse and, 

 secondly, longitudinal, tubercles appear, and finally longitudinal bands. 



Beecher,^ studying spines, found that they only appear after the 

 earlier stages, and he suggested that they may be formed by a special 

 stimulus given to the secreting organs. Sometimes they are due to 

 radiating lines being broken, and differentiated into spines, as in 

 Lima. In such forms as the Murices and Spondyli he states that they 

 are often formed by local repetition close to or on an ordinary spine. 

 Further, he calls attention to the fact that in some of the Pliocene 



^ Ostroumoff : Zool. Anz., vol. xxiii, p. 499. 



* Amer. Nat., vol. xxxvii, p. 481 ; cf. Dall, Tert. Fauna Florida, pt. v. 



' Proc. Amer. Ac, vol. xxxix, p. 123 ; Amer. Nat., vol. xxxiv, p. 863 ; Science, 

 vol. XV, p. 531 ; Journ. Exper. Zool., vol. i, p. 607, etc. 



* Zeitsch. Wiss. Zool., vol. Ixi, pp. 261-317. 



* Amer. Jouru. Sci., ser. iv, vol. vi, pp. 1-20, etc. 



