Dr. J. E. Gray on some Families of Bivalve Shells. 369 



quadrangular, subequivalve, twisted ; the left valve largest and 

 more twisted, with the hinder slope more or less keeled ; margin 

 smooth or subcrenate; cardinal area narrow, grooved; cardi- 

 nal teeth gradually broader at the distal end, and divided into 

 large obHque plates. Periostraca paleaceous, brown. 



* Hinder slope of left valve keeled. T. tortuosa. 



** Hinder slope of left valve rounded. T. semitorta. 



4. Barbatia, Gray, Syn. B. M. 1840, 155. Shell oblong, lon- 

 gitudinal or subquadrate, equivalve ; umbo subcentral ; cardinal 

 area narrow, angularly concentrically grooved; cardinal teeth 

 curved, at the outer end dilated and divided into broad, more 

 or less oblique or subconic plates. Periostraca paleaceous, with 

 more or less elongate, hair-like, or foliaceous projections. 



a. Periostraca thicks with elongate hair-like lobes in the radiating 



grooves, with a furrow along their upper edge ; shell brown, 

 solid. B. fusca (Australian), B. barbata (Mediterranean). 



b. Periostraca thin, with slender hair-like filaments in the radiating 



grooves ; shell thin, white. B. parva. 



c. Periostraca paleaceous, brown, with broad, fiat, foliaceous lobes 



on the edge ; shell thick, white. 



B. Helblingii = A. decussata = A. velata. 

 B. obliquata = A. Sinensis, B. fasciata. 

 B. lactea, B. raridentata, B. tenebrica. 

 B. glacialis. 



d. Periostraca thin ; shell white, cancellately ribbed or coS' 



tated; hinder slope subcarinate. Acar, B. reticulata, B. diva- 

 ricata, B. gradata. 



e. Periostraca thin, smooth, with a series of triangular fan-shaped 



appendages in the radiating grooves; shell white; hinder 

 slope strongly keeled; front and hinder margin dentated. 

 Calloarca. B. alternata. 



The plates into which the cardinal teeth are divided differ 

 considerably in specimens of the same species, not only in size 

 and number, but also in form and direction ; this is particularly 

 the case with the specimens of B. glacialis from the Arctic 

 Ocean, a species so distinct in form, substance, periostraca, and 

 habitat, that there can be no difficulty in regarding all the varie- 

 ties as belonging to the same species ; yet it presents such vari- 

 ations in the form and character of the teeth, that one might be 

 led, from only one or two specimens, to separate them into 

 different subgenera. For in some the laminse of the outer end 



Ann. ^ Mag, N, Hist, Ser.2. Vol.xix. 24 



