INTIMATE STRUCTURE OF THE SHELLS OF BRACHIOPODA. 33 



Many of these might probably be distinguished from each other by the size and 

 arrangement of the shell-canals ; but until it shall have been determined, by the examina- 

 tion of numerous specimens, what are the limits of variation in this respect, it would 

 be unsafe to employ this character for the diagnosis of species, except where a very 

 strongly-marked difference presents itself. One of the most remarkable departures from 

 the usual regularity of their arrangement, is presented by Ter. coardata, the surface of 

 whose shell is covered with square or oblong elevations, arranged in rows, and occasionally 

 running together into ridges; for the canals usually pass towards the highest parts of 

 these elevations, so that their orifices are there crowded together, whilst in the interme- 

 diate furrows there are scarcely any to be seen. 



Terebratulina, — Recent, abyssicola, angusta, cancellata, Candida, caput-serpentis *} 

 Cumingii, japonica, septentrionalis. 



Terebratella, — Recent, Bouchardii, cJdlensis, coreanica, Cumingii, dorsata, Evansii, 

 Jlexuosa, inconspicua, labradorensis, rubella, rubicunda *, sanguinea, spitzbergensis, 

 zelandica ; — Fossil, hamispherica ? 



Magas, — Fossil, pumilus. 



BoucHARDiA, — Recent, tulipa *, Blainv. zrr (ro^m. Humph.) 



Kraussia, — Recent, cognata, Beshagsii, Lamarckiana, pisum, rubra *. 



Megerlia, — Recent, pidchella *, truncata * ;^ Fossil, lima} 



The greater part of the internal surface of the shell Megerlia truncata is studded with 

 httle papillary eminences, on which the shell-canals do not open. (Plate IV, fig. 14.^) 



Morrista, — Recent, anomioides^ {depressa, Forbes.) 



Argiope, — Recent, cisiellula, cuneata, decollata, lunifera, neapolitana ; — Fossil, 

 decemcostata. 



Strigocephalus, — Fossil, Burtini. (See p. 31.) 



Ge7ms — Thecidea. 



In this genus — which, if associated with the Terebratulida at all, must be considered 

 as a very aberrant form of that family — the structure of the shell is much less character- 

 istic than it is in the Brachiopoda generally, and much less distinguishable from that of 

 ordinary bivalves. From appearances which I have occasionally detected, I am inclined to 

 believe that the shell is originally formed on the same plan with that of other Brachiopoda, 

 but that the component prisms subsequently coalesce, so as to form a more compact 

 texture. The shell-canals are very obvious in the recent Thecidea mediterranea *, opening 



' PL IV, figs. 11-13. 



2 By a mistake on the part of the artist, these eminences are not made to exhibit the imbricated 

 arrangement of the extremities of the prisms, which they present in common with the remainder of the 

 internal surface of the shell. 



8 PI. V, fig. 2. 



5 



