82 ntOCEEDlNGS OF THE MALACOLOGICAT. SOCIEXr. 



Genus Pkotothaca, Dall, 1902.. 



Type, Chama tJiaca, Molina (= Venus domheyi, d'Orb.). 



This group of shells was separated by Dr. Dull in his " Synopsis of 

 the Veneridae ",' and placed as a sub-genus of Tapes. I have protested 

 against this allocation on two previous occasions, holding that the 

 species which he took as his type, i.e. that usually known as Vetius 

 dontbeyi, is much more closely allied to Vemis and Chione than to 

 Tapes. A careful examination of all the species which Dr. Dall then 

 included under the name Protothaca lias convinced me that they do 

 not form a homogeneous natural group, but a heterogeneous assemblage. 

 One of the species belongs to the typical section of Chio7ie, another is 

 an abnormal form of Tapes; but the rest (including the type) do 

 ])reseut peculiarities which distinguish them botli from Chione and 

 Tapes, and possess characters which make it inconvenient to class 

 them as a sub-genus of either. 



The fact is that Dr. Dall's diagnosis only records some of the 

 ditferences between Protothaca and Chione, and those are chiefly 

 superficial differences. He dwells chiefly on the external sculpture, 

 and does not say a word about the disposition of the teeth, nor does 

 he sufficiently distinguish the group from Tapes. The sculpture, 

 being partly concentric and partly radial, differs little from that 

 ])ossessed by the typical section of Chione, and would not entitle the 

 shells to more than sectional value, but there are points of much 

 more importance, and one of these is the closer approximation of the 

 teeth. No doubt this was perceived by Dr. Dall, and was the chief 

 reason for his placing the shells under Tapes, although he does not 

 say so, nor does he distinguish Tapes from Chione by the divergence 

 of the teeth. 



It is a fact, however, that in Protothacahoih. the posterior cardinals 

 are shorter than in Chione, the left posterior being a short oblique 

 tooth crossing the hinge-plate on a line nearly parallel to the hinder 

 side of the median, while in Chione it is a long tooth, parallel to the 

 base of the ligament. Protothaca resembles Chione in having a strong 

 hinge-plate, nnd consequently there is a space between the left 

 posterior cardinal and the base of the liiiameiit. The ligament itself 

 is very long, extending nearly to the end of the posterior dorsal slope, 

 the consequence being that the groove, which is usually found on this 

 margin of the right valve, is in Protothaca merely a short indentation 

 for the reception of an equally short projection on the left valve. 

 In this respect it differs from Chione, and resembles some forms of 

 Tapes, such as 2\ deciissatus and T. ptdlastra. 



The following is a list of the recent species which are referred to 

 Protothaca and its section Callithaca by Dr. Dall : — 



Chione grata, Say {= Venus discors, Sow., and V. histrionica, 



Sow.). 

 CJiama thaca, Molina (= I'enus donihei/i, d'Orb.). 

 Chione ruder ata, Desh. 



' Proc. U.S. Nat. Mns., vol. xxvi, p. 364, 1902. 



