72 SHELLS AND SIIELL-FISH. PART 1. 



thickened in the middle, so as to form a slight elevation; 

 and in one genus (Rici)iula), both lips being toothed 

 renders the aperture very narrow : they are all heavy, 

 thick, and rough shells, often armed with tubercles and 

 spines, but never with varices ; there is generally an in- 

 ternal groove at the top of the aperture. The true type 

 of the whole sub-family, which is a representation of the 

 Cyclohranchia, is Concholepas, — a most singular-formed 

 shell, which was long classed as Patella : next to this 

 follows Monoceros, having a basal spine at the aperture 

 much developed. Following these, we place certain 

 shells where the basal notch is so small as to be nearly 

 obsolete. Of all the different modifications in the genus 

 Purpura, as it now stands, we think these point out the 

 third sub-genus, since they are evident representations 

 of Cerithuni, Melania, and other half-emarginate groups: 

 the spire is very short, and the pillar curves inwards. 

 Following these, which we distinguish by the name of 

 Microtoma, come the ordinary PurpurcB; and these 

 blend imperceptibly into Riciyiula, by means of P. coin- 

 mellaris, and Ricinula aspera and morus.* 



(QS.^ Ricinula is a most remarkable genus of small 

 prickly shells, with a very short spire, and the mouth, 

 in the typical species, so beset on both sides with tooth- 

 like projections, that the aperture itself becomes remark- 

 ably narrow ; but these teeth are only developed at the 

 matured age of the animal, and even in some species 

 they are very small ; nevertheless the pillar is almost 

 always flattened. The gradual developement, and some- 

 times the digitated outer margin, of the external lip, 

 shows a strong analogy to the Stronibidte, while the 

 toothed aperture instantly reminds us both of Cohunhella 

 and Cassis : all these relations will be still more deter- 

 minate, if the circle of the Purpiirince is tested on the 

 same principles as those last examined. It is this group, 

 in fact, which unites the Purpurince with the NassincSy 

 by means of Vexilla, where we see in the toothed outer 



* Ency. Meth. pi. 317. tig. 4. 6. t Ibid. pi. 317. fig. 5. 



