CERIOX, GROUP XIII. 271 



has not the expanded umbilical area of C. reglna eucosmium, of 

 Turk's Island. 



White specimens of this variety are very similar to C. eleut/ter&, 

 'but do not taper gradually as that species, the angle of obliquity of 

 the aperture is different, etc. 



Several hundreds of this species were obtained by Mr, John Ford 

 from a barrel of shells from the Bahamas, exact island unfortunately 

 unknown. On comparison with the nearly complete series of Cerion 

 in the collection of the Academy, it is evident that a new poly- 

 morphic species is before us, probably from an island or region of 

 an island hitherto unexplored for this genus. Both the striate and 

 smooth forms occurred either white or streaked, and so far as we can 

 judge in nearly equal numbers. Transition forms are fully repre- 

 sented, though probably 95 per cent, of the specimens are either the 

 one or the other. 



Figures 71, 72 are examples of variation, selected as the most ex- 

 treme uninjured specimens among some hundreds of shells examined. 



Subgenus PIACERION Dall, 1894. 

 Diacerion DALL, Bull. Mus. Comp. Zool., xxv, p. 122. 



The inner end of the rather short parietal lamella is contiguous to 

 or continuous with a denticle, or a long, spirally-entering lamella; an 

 infra-parietal denticle or parallel lamella is developed between the 

 parietal lamella and the axis. Part or all of these lamellae are some- 

 times absent. Axial fold developed. Shell ribbed. Type C. dalli. 



This division contains two groups of species : those of Great 

 Inagua, Bahamas, in which the lamella? may be very long inside, 

 and those of southern Cuba, in which, so far as we know, three 

 comparatively short lamellas are invariably developed, or the ''pari- 

 etal" and the k ' spiral" lamella; may be united into one long lamella. 



XIII. Group of C. rubicundum (Great Inagua). 

 The internal armature typical of this group is shown in pi. 45, fig. 

 88 ; but by shortening of the spiral (s) and infraparietal (i) lamellae 

 a condition exactly like the Cuban group of C. striatellum is some- 

 times produced (fig. 98). Further reduction leaves only the parietal 

 lamella (pi. 4(, fig. 10); and in C. bryonti even this may be lost. 

 These stages of tooth development are demonstrated by the series 

 before me to be in no way due to age ; I speak of fully adult shells. 



