728 /. HOWARD WILSON 



hamatus by collectors from this locality, and all listed under the 

 latter name. 



In regard to the position in the section of Venus mercenaria var. 

 antiqua Verrill, previous reports mention it as abundant in the "lower 

 shell bed," but not found above this horizon. The writer found, 

 on the contrary, that the most typical forms occurred only above 

 the serpula bed, in No. n, while forms less typical, and more inter- 

 mediate between it and the common species, were found in the 

 serpula bed. In this case, the name antiqua would be somewhat 

 of a misnomer, as the variety would seem to represent simply a short 

 lived mutation from the common species. 



The Astartes as a group are very variable, presenting some puz- 

 zling differences within what may be considered a single species. 

 Several species occur in the Sankaty deposits, and a small form 

 which occurs very abundantly in the upper beds (over two hundred 

 being collected), seems to have been identified in the past as Astarte 

 quadrans Gould; but of the great number collected, the one which 

 approaches nearest to A. quadrans of Gould seems to be identical 

 with the variety portlandica of Mighels, the others differing from A. 

 quadrans in a line of development far beyond even A. portlandica. 



A number of the Sankaty Head forms are shown in Fig. 11, Nos. 

 6-15, while several A. castanea from the same locality are figured 

 in Nos. 1-5 for comparison. The whole series of A. quadrans differs 

 from A. castanea in their small size, excentric position of the beaks, 

 the nearly straight anterio-dorsal margin, the absence of the broad, 

 slightly elevated bands of growth sometimes found in A. castanea, 

 and the fact that the latter has a much heavier shell, with very much 

 higher hinge area and stronger teeth. 



In the figures shown, No. 14 is almost identical with Gould's 

 figure of A. portlandica, and is considered here as representing the 

 type of that species. Most of the other forms figured, however, 

 seem to show specific differences, being much higher and having 

 the beaks much more excentrically situated, and should perhaps be 

 separated as a distinct species, for which the name sankatyensis is 

 proposed. 



Nos. 7-12 exhibit very well this great divergence from the type 

 of A. portlandica, while Nos. 6, 13, and 15 are intermediate types. 



