THE NAUTILUS. 103 



NOTES. 



Note on Thyasira bisecta Conrad. — In 1889 I gave an ac- 

 count of the microscopic anatomy of a species of Thyasira (under 

 the name of Cryptodon) in my report on the Blake dredgings, 

 p. 438. This was I believe the first general account of the 

 unique features of this genus, the data on Cryptodon furnished 

 by Pelseneer in the Challenger report relating to Lyonsiella or a 

 similar genus rather than to Thyasira. Pelseneer himself re- 

 ferred them to ^^ Cryptodon ^^ with doubt. The specimen de- 

 scribed in the Blake report was 17 mm. high, and regarded as 

 exceptionally large. The species referred to Thyasira, under 

 the name of bisecta Conrad, was so placed by me because of its 

 agreement conchologically with that genus, although it had been 

 referred to several different genera and a new genus had been 

 proposed for it by Gabb. 



I had long been anxious to examine the anatomy of this 

 mollusk, which reaches a height of 75 mm., to see whether it 

 conformed to the primitive features of the small typical forms 

 of the genus, and by the kindness of Mrs. Oldroj-d and Dr. 

 Frye of the Friday Harbor Biological Station, Puget Sound, 

 this wish has been granted. The specimen was found in about 

 four fathoms, muddy bottom among the San Juan Islands. 



Rather to my surprise I find that the description written of 

 the small species nearly thirty years ago applies almost word 

 for word to this giant of the genus. The only difference seems 

 to be the greater proportional length of attachment of the W- 

 shaped gills, and the only addition is the presence of a glandular 

 area within the basal edges of the mantle extending nearly the 

 whole length of the free edges. Something of this sort might 

 have occurred in the earlier specimen but have been overlooked 

 on account of its minuteness. The arborescent hepaticogenital 

 organs occupy the greater part of the mantle cavity, while the 

 absence of papillae on the mantle edge and around the efferent 

 aperture in the mantle, and of oral palpi, the worm-like foot, 

 etc., are essentially the same as in the smaller forms. 



Geologically, T. bisecta recedea to the Miocene. — W. H. Dall. 



