MAZATLAN BIVALVES 45 
Although there is little doubt that Valenciennes’ species is 
the rayed variety of this shell, and therefore has priority, yet 
as his description is not accurate enough for certainty, and 
as it only applies to a very small part of the species, Hanley’s 
expressive name is retained. The punctatostriate character 
however is by no means peculiar to this shell. Some forms 
are exceedingly hard to tell from D. Conradi, Desh. ; including 
D. contusa, Rve. The following are believed to be the main 
characters of the shell. Surface smooth, shining, with very 
thin deciduous epidermis, rarely seen. Not grooved, but with 
rows of extremely fine dots indented as with a bookbinder’s 
tool. Rows often intercalated near the anterior end. Mar- 
gin with strong squarish crenations inside: the same outside 
toward the posterior slope; about the middle, grooves appear 
dividing each, answering to the intercalary rows: at the end 
a broad surface is channeled off, in which each crenation gives 
off a fang something like a (*) to join the external rays. This 
intricate and very characteristic structure will generally tell 
the species at once from the aberrant forms of D. Conradi. In 
the posterior ‘slope, the punctate rows are very close; in the 
lunular portion entirely absent. The ordinary colour is pale 
yellow or white, sometimes arich orange yellow; generally 
rayed with purple about the lunule with occasional stains 
elsewhere ; often tinged with green about the posterior slope 
and umbos, occasionally rayed with brown (=D. radiata, Val.) 
with the rays often not corresponding on the two valves. 
Umbos white or pale yellow, sometimes rich transparent orange, 
sometimes with a penciling of purple, sometimes two purple 
spots. Shape normally trapezoid, with the umbos sub-central : 
but often with the posterior part shortened, and the anterior 
much elongated. The end is then tapering: but often it is 
short and very gibbous. Ventral margin almost always well 
developing the angular swelling. Inside generally rich purple. 
Lateral teeth short but very strong, the anterior truncated : 
in the opposite valve small. 
Largest specimen measures Jong. 1°15, lat. 1°64, alt. °6. 
Transverse ,, as i NS ae he, MOS W estas Slee 
Stumpy 29 ” oe) ; 79, ” i 06, sos 48. 
Smallest s * i MEG? Sosa lds. cai woe 
Hah—South America, Capt. Ld. Byron |? |.—Mazatlan ; in 
extreme profusion ; L’pool Col. 
Tablet 142 contains 15 specimens, normal shape, rich orange 
yellow.—143, 10 sp. same colour, transverse shape.—144, 9 sp. 
