94 



INTRODUCTION. 



plain, the lower cirrated; both are united at the sides of the mouth, and form long 

 appendages (or lahial arms), coiled up spirally, with their ends directed inwards towards 

 the cavity of the dorsal valve. The alimentary canal takes the same course as in 

 Ter. australis, passing through the deeply-notched hinge-plate, and ending behind the point 

 of insertion of the adductor muscle in the centre of the ventral valve. The pallial veins 

 (like those oi Bh. acuminata, p. 95, figs. 33, 34, and Camarophoria, p. 97, figs. 35, 36,) are 

 much narrower than in Terehratida, and more angular in their mode of bifurcation ; there are 

 four principal branches in each lobe, opening into large sinuses similar to those of Orthis 

 and Strophotnena, but smaller in extent. The margin of the mantle is fringed with a few 

 short horny setcB. The muscles are essentially like those of Terebratula ; their impressions 

 are referred to in the description of the shell. Shell inequivalve, variable in shape, trans- 

 verse or elongated, circular or trigonal ; valves more or less convex, with or without a 

 mesial fold and sinus ; beak entire, acute, prominent, or so much incurved as to leave no 

 free space for the passage of pedicle muscles ; foramen variable in its dimensions and 

 form, placed under the beak, exposed or concealed, entirely or partially surrounded by a 

 deltidium, the aperture being sometimes completed by a portion of the umbo of the 

 smaller valve ; deltidium in two pieces, at times either extending in the form of a tubular 

 expansion, or rudimentary; surface striated or plaited, rarely smooth ; structiu'e fibrous, 

 impunctate. Valves articulating by means of two teeth in the larger (ventral), and 

 corresponding sockets in the imperforated (dorsal) valve; apophysary system, in smaller or 

 dorsal valve, composed of two short, flattened, and grooved lamellse, separate and 

 moderately-curved upwards, attached to the deeply divided hinge-plate ; in the socket 

 valve the quadruple impression of the adductor muscle is clearly defined, and separated 

 by a short medio-longitudinal ridge {s) ; the pedicle scars occupy the small cardinal plates, 



between which is the small and narrow 



Fig. 30. 



Fig. 32. 



Fig. 31. 



Rh. j)sittacea. 



cardinal process. In the perforated 

 valve the two strong diverging car- 

 dinal teeth are supported by dental 

 plates extending to the bottom of the 

 valve, and at the base of these a semi- 

 circular ridge curves on each side, 

 forming a more or less defined saucer- 

 shaped depression, into which were 

 affixed the shell and pedicle muscles; 

 these last leave two narrow elongated 

 scars, close to the inner base of the dental laminae, the remaining and largest portion 

 being chiefly occupied by the cardinal muscles, which are longitudinally divided by a small 

 raised ridge ; above these again is seen a small oval scar due to the adductor. 



Kh. nigricans.^ 



a, adductor, 

 i, intestine. 



f, foramen. d, deltidium 

 t, teeth. a, adductor, 



p, pedicle muscles. o, 



c, crural processes. 



t', sockets. 



r, cardinal. 

 0, ovarian .spaces, 

 s, septum. 



From Mr. Evans's specimen, see note to p. bj- The original was very unsymmetrical. 



