141 



It is rarely that colour presents any character of sufficient constancy to 

 be recorded as one of the distinguishing features of a genus. In the 

 present instance the shell is most inconstant in form, for whilst in one 

 instance it is of an ovately turbinated growth like a Nerita or Delpkinula, 

 in another it is of a sharp pyramidal form according to the true type of 

 Trochus. In all cases the shell is of a peculiar opake cream-white sub- 

 stance, more or less stained with dark brown, and it is a permanent 

 character of the columella to be furnished with a sharp lamelliform or 

 shelf-like tooth. One of these forms was collected by Mr. Cuming at the 

 Philippine Islands, the other at Panama. 



For List of Species see Appendix. 



Figure. 



Morulxjs cidaris, PI. 13. Fig. 63. Shell, showing its dull opake sub- 

 stance and lamellated tooth upon the columella. — From Mr. Cuming's 

 collection. 



Genus 12. ROTELLA, Lamar ch. 



Animal ; ? 



Shell ; orbicular, smooth, polished, spire very short, base callous, 

 about equally convex with the spire, aperture semicircular. 



The Rotella form another interesting section of the Trochus tribe, of 

 small size, and easily recognised by their lenticular shape and polished 

 surface. The species selected for illustration is one of extreme rarity and 

 extraordinary large size ; few exceeding a half to three quarters of an inch in 

 diameter. It is curious to observe that the Rotetta, with their polished 

 exterior, are never found with any foreign matter attached to them, and 

 M. Deshayes thinks that this indicates a peculiar organization of the 

 animal, in which the mantle is extended over the shell. 



Species. 



1. costata, Valenciennes. 4. gigantea, Lesson. 7. rosea, Lamarck. 



2. elegans, Beck. 5. Javanica, Lamarck. 8. suturalis, id. 



3. Guainensis, Quoy. 6. monilifera, id. 9. vestiaria, Sowerby. 



