362 Mr. T. Davidson on the Classification 



tlie British Museum. My only object is to point out how all the 

 sixty-six or sixty-seven known species may be classed according to 

 their structure and affinities. In drawing up any scheme for 

 the arrangement of the Brachiopoda, due regard must be paid 

 to the extinct species^ which vastly outnumber those of the 

 present seas. The family Rhynchonellidae for example afford 

 only two living species, whereas several hundred are known in a 

 fossil state : and in the same manner the proposed subdivisions of 

 TerebratulidcB become more important in reference to the number 

 of species which no more exist. When we open and examine the 

 numerous species possessed of calcareous appendages, we observe 

 that these vary in shape and arrangement, and that under each 

 form or type a number of species can be clustered all very similar 

 in their structure ; these form genera or subgenera, more or less 

 intimately or distantly connected, as will be shown hereafter, and 

 thus enable any one to know by the generic appellation, what 

 is the form and disposition of the skeleton or loop in all the 

 different species. The subdivisions of the great genus Tere- 

 hratula, are based chiefly on the extent and form of the apophy- 

 sary system or loop, and my object has been to ascertain how far 

 the form of the loop can be associated with the modifications in 

 the soft parts of the animal and in the more obvious external 

 characters presented by the shell. The woodcuts which accom- 

 pany this paper will serve to render the generic descriptions 

 more complete and readily understood. 



It can scarcely be expected that this first attempt to classify 

 the recent species of living TerebratulidcB should be entirely suc- 

 cessful, but such a classification has become not only desirable 

 but necessary, and no good would be accomplished by delaying 

 its publication. With these few preliminary observations I will 

 propose the following arrangement. 



Class BBACHIOPODA, Bumeril, 

 Order Lamellibranchia.ta, Blainv. 



Animal attached to submarine bodies by a muscular peduncle 

 or by the substance of its ventral valve : furnished with a pair of 

 ciliated arms, sometimes supported by a calcareous appendage ; 

 respiration performed by the vascular mantle, &c. 



Typical Family. I. Terebratulid^. 



Shell minutely punctate, inequivalve, usually subcircular and 

 smooth or striated: the ventral or dental valve generally the largest, 

 its beak notched or perforated for the passage of the peduncle : 

 dorsal, socket, or receiving valve, furnished internally with a cal- 



