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, Desk. ; including 

 D. contusa, jRve. 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 appeal- 

 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 a rich 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, J r ctl.) 

 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 long. 1*15, lat. 1*64, alt.'S. 



Transverse '96, 1'46, '57. 



Stumpy *79, 1'06, '48. 



Smallest *18, '26, '11. 



Hob. 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. 



