OPEAS. 123 



Opeas and Curvella is 'by no means convincing ; yet even small 

 differences have significance which, may usefully be recog- 

 nized in dealing with large groups of similar species. 



Opeas and SubuUna begin to reproduce before the shell has 

 attained its full size, usually when it is about two-thirds 

 grown. 



Dimorphism. In many species two forms co-exist in the 

 same colony, a more slender and a stouter ; all other features 

 remaining the same. Intermediate contours usually occur if 

 a large gathering is at hand. See plate 16, figs. 89, 90. This 

 dimorphism must be kept in mind, especially when dealing 

 with small sets. 



The species are here separated geographically into (A) 

 Old World and (B) American species. 



(A) OLD WORLD SPECIES. 



Excepting a few forms spread by commerce or other means, 

 the Old and New World species are distinct. Most of the 

 Eastern forms belong to typical Opeas. 



Several minor groups are indicated by slight conchologic 

 features. In one group which may be called Tomopcas, n. 

 sect., the columella is calloused below and subtruncate. Type 

 0. layardi (pi. 16, fig. 83) ; also 0. avenaceum, 0. soror, 

 0. brevior. 



In another group which I term Comoropeas, n. sect., the 

 shell is wholly imperforate, covered with a colored cuticle; 

 first whorl rounded, the rest rather flattened, the first two 

 smooth. Type 0. apiculum Morel. ; 0. longulum also belongs 

 here. These forms may not belong to the genus Opeas ; they 

 seem related rather to the Trichodinas of the Comoros (see 

 vol. XVII, p. 188) ; yet they have not trochoidal apices like 

 that group, but an obtuse, rounded summit, much as in Opeas. 



Another series may eventually be transferred to Prosopeas, 

 including the species javanicum, pilosum, pruinosum, kus- 

 aiense and fagoti, characterized by the densely striate and 

 briefly lamellose surface. 



A single species, 0. carinatum, no. 38, is described as with 



