174 DR. T. DAVIDSON ON EECENT BEACHIOPODA. 



raarked only by some fine concentric lines of growth. Shell-structure composed of 

 numerous close-set microscopic imbricated scales. There is a small cardinal process seen 

 in the interior of tlie dorsal valve, from the base of which extends, to about half the length 

 of the valve, a mesial septum or large vertical blade-like plate. This rises gently by an 

 upward curve until it attains its higiiest elevation, then presents a short horizontal line, 

 to be again abruptly curved inwards until it reaches the bottom of the valve. On each 

 side of the cardinal process a narrow, small hinge-plate is seen, from which extend two 

 short, slender, curved lamellae denticulated at their extremities ; and at the bottom of the 

 valve on each side of the septum well-defined muscular scars are situated. In the 

 interior of the ventral valve two short but strong, diverging dental plates are joresent. 

 The valves are feebly articulated by means of small teeth in the ventral valve and 

 sockets in the dorsal one. Length 2i lines, breadth 2, depth 1 line. 



Hab. This species was di-edged by Dr. Gwyn Jeffreys in Davis Strait, during the 

 'Valorous' Expedition, at depths between 1100 and 1750 fathoms, and during the 

 ' Porcupine ' Expedition, in from 1380 to 1443 fathoms ; and during the Norwegian North- 

 Atlantic Expedition, at about thirty miles west of Tromso, on the slope of the banks in 

 the cold area in 650 fathoms (Eriele). 



A single living specimen occurred in lat. 63° 9' N., long. 56° 43' W., at a depth of 1100 

 fathoms. Valves and fragments were also found in lat. 59° 10' N., long. 50° 25' W., and 

 in lat. 66° 11' N., long. 37° 41' W., at depths of 1750 and 1450 fathoms. Imperfect 

 specimens had been dredged by Dr. Jeffreys during the ' Porcupine' Expedition of 1869, 

 at Stations 20 and 30, at depths of 1443 and 1380 fathoms, off the west coast of Ii*eland. 

 Atretia gnomon was also obtained by the ' Talisman ' Erench Expedition off Morocco 

 and the Canaries, at dejiths of from 50 to 65 fathoms. 



Obs. Since publishing my restored figures of this small species in the Paheontogra- 

 phical Society's volume for 1874, from ten to fifteen * complete examples have been 

 obtained by Dr. Gwyn Jeffreys and by the Marquis de Eolin, and I have been able 

 to ascertain that my figures, restored as it were, were quite correct. None of the 

 examples exceeded the dimensions given. The presence of the internal septum is indi- 

 cated on the exterior of the dorsal valve by a dark line. Dr. Gwyn Jeffreys observes in 

 his paper published in 1878 : — " According to Mr. Dall, Atretia is a synonym of Ziltel's 

 genus Dimerclla (Dunker and v. Meyer, Palreont. 1870, p. 220), which is a Jurassic Bra- 

 chiopod ; but, although it belongs to the same family, the internal apparatus is different, 

 and the beak is prominent in Atretia and incurved in Dimerella. The septum in Atretia 

 is short, gnomon-shaped, and central ; in Dimerella it extends from below the beak to 

 the front of the lower or smaller valve. The genus Mannia of Dewalque (Soc. Malac. 

 Belgique, 1874), from the ' sable noir,' or Upper Miocene of the Antwerp Crag, is also a 

 Rliynchonellidan ; but the internal apparatus is likewise different from that oi' Atretia." 



* [About fifty specimens of Atretia gnomon were dredged by Dr. Herman Friele during the Norwegian North- 

 Atlantic Expedition, but no species of BhpicJionella occurred at the same station or in corresponding depths. — A. C] 



