MOOEE — ON NEW BEACHIOPODA, ETC- 



441 



Mr, Davidson. It 'will be seen from wliat follows that two species at 

 least of Terebratella are to be found in tbese beds ; but when D'Or- 

 bigny placed the Terebratula liemispiicBrica in this genus, he could 

 not have seen its interior, which, in several examples I possess, have 

 the short and simple loop of Terebratula, and the shell in question 

 will therefore have to return to its original position. 



Teeebratella. 



Terebeatella BtJCEMANii. Woodward, MS. PI. xiii., figs. 1-5. 



Shell generally a little longer than wide, rounded in front, and 

 tapering to the beak ; valves moderately convex ; beak short, very 

 slightly incuiwed and truncated by a foramen, surrounded in part by 

 the extremity of the beak, the umbo of the dorsal valve, and two 

 small labral deltideal plates. Internally the adult shell is provided 

 with a doubly attached loop, the first pair of lamellss extending con- 

 siderably before becoming reflected to form the loop. Shell struc- 

 ture punctuate. Dimensions of the largest example hitherto 

 observed : length, 3| lines ; width, 3 lines ; depth, 1^ lines. 



Ohs. — I had collected a considerable number of these little 

 brachiopods from the oolite of Hampton Clifis, under the idea that 

 they were the young of the Terebratula maxillata, my object being 

 to prepare dissections showing the loop of that species. I was much 

 interested in finding in the example I first opeued that it could not 

 belong to the young of that genus, although outwardly it is almost 

 undistinguishable from it. The difference in the loop proved it to 

 be a true Terebratella. My observation does not show that it 

 attained larger dimensions than those mentioned, but it had then 

 assumed the character and development peculiar to the loop which 

 characterizes the genus to which it is referred. In an early stage of 

 my examination, modifications in the shape of the loop were noticed, 

 and observations extending to several hundred specimens resulted in 

 showing the curious changes effected by age in the form of the loop, 

 which may be seen by referring to plate xiii., figs. 2, 3, 4, and 5. 



The first stage of development I have been able to observe is 

 sketched in fig. 2. Therein it may be perceived the two first lamellse 

 are united to the hinge-plate, and to a free rudimentary mesial plate, 

 which is, in fact, the first origin of what at a later period becomes a 

 mesial plate. In this state it is free, and does not touch the bottom 

 of the valve, although when viewed in profile spines may be seen 

 passing downwards, which afterwards join the mesial septum. 



The loop has not yet been formed, but a plate projects between 

 the lamellae, and appears as if longitudinally split to a certain depth 

 in the centre. 



The second stage is exhibited in fig. 3. In this we find the two 

 lamella with the rudimentary plate as in fig. 2, and, besides, the 

 origin of the reflected portion of the loop, presenting in this first 

 stage of its development but a very small a-nd rudimentary aspect. 



VOL. III. 3 K 



