''"isga"'] PROCEEDINGS OF THE NATIONAL MUSEUM. 215 



is frequently iucouspicuous and feeble. The vah^es are rather thiu and 

 somewhat trauslucent, bluish white ou the iuside and showing the ribs 

 when held up to the light. 



Dimensions: From umbones or beaks to opposite edge 15'"'", from 

 anterior to posterior edges 15'"™, varying the fraction of a millimeter 

 in either or both of these dimensions in dift'ereut individuals. 



Habitat. — Station 2840, off Station Barbara Islands, California, in 

 green mud at 276 fathoms depth ; U. S. Fish Commission steamer Alba- 

 tross, May 8, 1888, Very abundant ; several hundred specimens were 

 obtained (U. S. National Museum, No. 104015). 



A comparison of this shell with Gould's ventricosa (PI. xvi. Figs. 5, 

 6), shows not only a great difference in the elevation of the beaks and 

 form (out line) of the valves but in the characters of the hinge, as well as 

 the thickness of the hinge plate. In ventricosa as well as in horealis and 

 Miodon prolongatus the, long solid posterior cardinal is strikingly con- 

 spicuous when compared with the same in harharensis, which is much 

 shorter, slighter, and without curve; the anterior cardinal is solid and 

 thick with a somewhat diverging curve {i. e., curving away) from the 

 posterior cardinal. 1\\ prolongatus and horealis the anterior cardinal is 

 triangular and solid and perpendicular to the point of the umbos, or 

 nearly so, with a hint in horealis of cleavage in said tooth, while in har- 

 harensis this tooth is acutely elongated and sinuously ovate and diverg. 

 ing anteriorly. Both ventricosa and horealis exhibit a small rounded 

 tuberculoid process anterior to the anterior cardinal, rather inconspic- 

 uous, and quite likely absent in some cases. This tubercle is suhmarginal 

 in these two species, and altogether absent in Carpenter's sheW prolon- 

 gatus^ if we may judge by the example figured (PI. xvi, Figs. 7, 9) ; while 

 in harharensis this character is seen to be, as elsewhere remarked, a 

 mr\\)\^ projection of the edge of the shell at the base or lower margin of 

 the lunule, varying in prominence, as before remarked, in difterent indi- 

 viduals. 



Venericardia boreaiis Conrad. 



Plate XVI, Fig. 8. 



This form was described by Conrad on page 39 of his American Con- 

 chology, of which only a few parts were published, and a figure given, 

 which shows the exterior of the right valve and an upper inside por- 

 tion of the left, with the hinge, the latter not as definitely figured as 

 is desirable. The description is quite brief and unsatisfactory when 

 compared with Dr. Gould's in the Invertebrata of Massachusetts, 

 wherein Arcturus rudis Humphrey MS, and Gardita vestita Deshajes 

 are included as synonyms. It is common in many places on the Atlan 

 tic sea-board, off shore, at various depths, from the Arctic sea to Hat 

 teras, at from 5 to 100 fathoms. Say's granulata, and Morse's nov 

 anglia', which Mr. Dall regards as varietal forms,* imply differences or 



* Vide Bulletin 37, U. S. National Museum, p. 46. 



