DE. T. DAVIDSON ON RECENT BEACHIOPODA. 77 



increasing in number at various distances from the beak and umbo by the interpolation 

 of shorter ribs between the longer ones, and crossed at intervals by concentric lines of 

 growth. Shell-structure punctate. Of a very light yellowish white or ochreous colour. 

 Length 1 inch 8 lines, breadth 1 inch 10 lines, depth 11 lines. 



In the interior of the dorsal valve the cardinal process and hinge-plate are well defined. 

 The loop is long and doubly attached, first to the hinge-plate, again by horizontal lamintc 

 given off from the principal branches of the loop to a slightly elevated mesial septum, 

 previously becoming deflected. 



Hab. Straits of Magellan ; Bay of Valparaiso, in depths of from 60 to 90 fathoms 

 (Cuming) ; Eoyal Sound, Kerguelen Island (' Challenger ' Expedition). 



Obs. This fine species has received several denominations, and some considerable dif- 

 ference of opinion has been expressed as to the name the shell should retain. The first 

 notices I can find of the shell are those of Davila in 1767, Favart d'Herbigny, Walchs in 

 1774, Favanne in 1780, and Chemnitz in 1785. These early naturalists had described and 

 figm-ed the species under the designation of the Magellan striated Anomia ; and as that 

 name is not binomial, Gmelin, in 1778, named it Terehratula dorsata, and, I believe, the 

 larger number of malacologists have preferred that designation, although some others 

 have insisted on retaining the name of Magellanica for the shell, which Gmelin should 

 have adopted. The names chilensis and Soioerbli are certainly synonyms, and L. Reeve 

 would likewise add to them the T.flexuosa of P. King. If this view were adopted, 

 one would have also to include the Terebratula rhombea, Philippi, and, in all probability, 

 the Terehratula htpinus of the same author ; but as these last-named species have large 

 Magasella-&\iii.\)eA septa and are adult, it may be better, at any rate provisionally, to locate 

 them in the subgenus MaQasella. I am the more inclined to adopt this view, as many 

 specimens of Terebratella dorsata, when compared with others of Magusella flexuosa, of 

 equal dimensions and age, can be distinguished by the absence of the elevated mesial septa. 

 Terebratella dorsata varies also somewhat in shape and striation ; young specimens 

 have their valves quite smooth, and even in some full-grown examples the umbo and beak 

 remain smooth, the ribbing commencing a little lower down. In 1867 Commodore Acton 

 dredged in the Straits of Magellan a very large number of specimens of Terebratella 

 dorsata, from the dimensions of one line in length up to twenty, and on his return kindly 

 presented me with a series of examples illustrating the different ages of the shell. 



When quite young, there exists in the interior of the dorsal valve a large and elevated 

 mesial plate or septum, from the anterior sides of which the principal stems of the loop 

 take their second attachment. Over these, but quite separate, are two small anchor- 

 shaped lamelltE, which are attached to the sloping portion of the septum facing the 

 hinge-plate. After a time these anchor-shaped lamellae become united, and their lower 

 edges are connected with those of the primary branches ; by degrees the principal stems 

 of the loop are separated again, become larger, and give ofi' a transverse branch, which 

 fixes itself to the upper anterior edge of the then low median septum. 



In 1847 A. d'Orbigny proposed his excellent genus Terebratella, giving the full-grown 

 condition of Terebratula dorsata, Gmelin, as his type. King's Terehratula Sozccrbil, from 

 the Straits of Magellan, is nothing more than a rust-coloured example of Terebratella 



11* 



