826 PROCEEDINGS OF THE NATIONAL MUSEUM vol.xxih. 



which is frankly lamellar on the ridges of the posterior dorsal area, 

 which are separated by a more marked radial sulcus and have the 

 whole area relatively wider than in P. crenella; in the interspaces only 

 is the radial sculpture of P. crenulatus visible and it is composed of 

 close-set threads usually uniform and rather strong; P. creneUa has 

 the concentric sculpture of low, very line threads or sulci which do 

 not conceal any part of the radial sculpture, which is feebler, less 

 compact, and more inconstant than in the Miocene shell, being fre- 

 quently almost entirely obsolete. The lunule in the two species is 

 similar, being larger and less impressed in the left than in the right 

 valve. The crenulation of the inner margin of the valves is stronger, 

 closer, and more prominent in the Miocene shell, in harmony with the 

 stronger radial sculpture. 



The specimen tigured is from Palma Sola. Florida, and measures 

 6.5 mm. in height, 0.7 in length, and 4.5 in diameter. The Lucina 

 crenulata of Searles Wood in the Crag monograph is a species belonging 

 to the same group, but apparently distinct from either of the Ameri- 

 can forms. 



PHACOIDES (PLEUROLUCINA) UNDATUS Carpenter. 

 (Plate XXXIX, fig. 14.) 



A figure of this elegant and hitherto unfigured species is now fur- 

 nished. The specimen shown is from the Gulf of California, and 

 measures 10 mm. in height, A larger size is common, but our fresh- 

 est and best .specimens are mostly only adolescent. 



PHACOIDES (BELLUCINA) AMIANTUS, new species. 

 (Plate XXXIX, fig. 10.) 



This is another case in which allied species have been indiscrimi- 

 nately lumped. Fortunatel}' the name costata, which has been gener- 

 ally used for it, is unavailable, leaving us free to name the component 

 species without regard to the original type of Tuomey and Holmes, 

 which is ditterent from that of Holmes in his Pleistocene volume, both 

 being very inadequately figured. Similar species occur from the Oli- 

 gocene to recent seas and on both east and west coasts of America. 



Shell small, solid, white, usualh^ subequilateral with strong sculp- 

 ture and hinge; beaks variable, usually rather conspicuous; sculpture 

 of about twelve strong, fiattish, radial ribs, separated by deep, nar- 

 rower, channeled interspaces, less distinct basally in senile specimens; 

 the ribs are crossed by numerous adjacent, flat, strap-like threads,which 

 in well-developed specimens seem to bridge the interspaces; dorsal 

 areas large and conspicuous; anterior with two broad wave-like radials, 

 sometimes slightly lamellose; posterior with one slender radial, which, 

 with the boundary ril) in front of the ar(>a, is conspicuously nodular; 

 lunule small, deeply impressed, \\\ defined; iiinge and nmscular impres- 



