THE BRACHIOPODA 5 



and were said to constitute a genus of animals. At the 

 present time the name Terebratula is used in a more 

 restricted sense, but we can still speak of all these forms 

 as terebratuloids. To distinguish these two species from 

 one another James Sowerby, nearly a century ago, named 

 one Terebratula intermedia and the other Terebratula obovata. 

 Let us now see some of the differences that underlie 

 their resemblances. 



The two Cornbrash species obviously differ in pro- 

 portions. The ratio of length, breadth, thickness is in 

 T. intermedia roughly 12, 10, 6, in T. obovata n, 10, 8, 

 the latter being thus broader and thicker. In the 

 latter also the boundary between the cardinal area and 

 the general surface of the ventral valve is a very definite 

 edge, while in the former it is rounded off so as to be 

 somewhat indefinite. The foramen of T. intermedia is 

 much larger in proportion to the size of the whole shell, 

 and the outer edges of the deltidial plates, if continued, 

 would form an obtuse angle ; while in T. obovata the 

 foramen is small, and the corresponding edges would 

 form a right or slightly acute angle. 



If we break open these two shells we shall often find 

 them to be hollow, and projecting into the cavity from 

 near the posterior end there is what looks like a looped 

 ribbon covered with little crystals of calcite. In the case 

 of T. obovata this loop extends forward nearly to the front 

 end of the cavity ; in T. intermedia it is less than half this 

 length (Fig. 3). In the former also there runs along the 

 middle line of the dorsal valve an internal ridge, forming 

 an incomplete partition (septum). The presence of this can 



