MUSEUM OF COMPARATIVE ZOOLOGY. 247 



beaks, not, as usual, oblique ; both ends rounded, the anterior rather the more 

 angular. Max. Ion. 5.1; max. alt. 4.0; max. diameter about 3.9 mm. 



Habitat. Yucatan Strait, in 540 fms., two right valves, one of which was a 

 little more triangular than the other. 



This little shell, which probably grows to a larger size, resembles a little 

 Corbicula or Astarte in its concentric, without any radiating sculpture. I have 

 not been able to find anything to which it might be referred, and, though the 

 material is scanty, have concluded to give it a name. 



B. Margin crenulated. 

 Nucula crenulata A. Adams. 



Nucula crenulata A. Adams, Dall, Bull. M. C. Z., IX. p. 123, 1881. 

 Nucula culebrensis E. A. Smith, Chall. Rep. Lam., 1. c, p. 228, pL xviii. figs. 11, 11 a, 

 1885. 



Plate VII. Fig. 2. 



Habitat. 20 miles west of the Florida coast, in 30 fms.; Station 36, 84 

 fms.; Barbados, 100 fms.; Sigsbee, off Havana, 158, 182, and 450 fms.; Station 

 20, 220 fms.; Station 19, 310 fms.; Yucatan Strait, 640 fms. 



Nucula crenxolata A. Adams, var. obliterata Dall. 

 Nucula crenulata, var. obliterata Dall, Bull. M. C. Z., IX. p. 123, 1881. 



Plate vni. Fig. 2. 



Station 44, 539 fms. ; Yucatan Strait, 640 fms. ; Station 2, 805 fms. ; Station 

 226, 424 fms., near St. Vincent; Station 236, near Bequia, in 1591 fms.; and 

 at Station 262, near Grenada, in 92 fms. ; bottom temperatures ranging from 

 39°. to 62°. F. 



This species is very variable, and presents sometimes an almost smooth sur- 

 face (as in the var. obliterata), and at others either a series of regular concen- 

 tric waves or a more or less broken and irregular concentric sculpture, the 

 whole being imited by every variety of transitional features. The most nearly 

 allied species is N. sulcata Bronn (not A. Adams), which is however less 

 trigonal, grows much larger, and yet has a finer and more irregular sculpture, 

 in which the concentric element is less dominant. The N. culebrensis of Smith 

 agrees so well with young, regularly sculptured specimens of crenulata, that, 

 taking the locality into consideration, I feel quite confident of their practical 

 identity. The ordinary adult and many young crenulata are more coarsely 

 and roughly sculptured, but this is not invariable, and the large number of 

 specimens I have examined have given an excellent opportunity for com- 

 parison. 



