226 



MANUAL OF THE MOLLUSCA. 



! 



by dental plates ; hinge-plate deeply divided, supporting oral lamellse, rarely \ 

 provided with spiral processes ; muscular impressions grouped as in Terehra- \ 

 tula ; vascular impressions consisting of two principal trunks in each valve, j 

 narrow, dichotomising, angular, the principal posterior branches inclosing \ 

 ovarian spaces. 



Animal (of Rhynchonella) with elongated spiral arms, directed inwards. < 

 towards the concavity of the dorsal valve; alimentary canal terminating behind ( 

 the insertion of the adductor in the ventral valve ; mantle not adhering, its { 

 margin fringed with a few short seta3. 



Ventral aspect. Umbonal aspect. 



Fig. 140. Rh. acuminata, internal casts. ' '• 



Fig. 14'>. Umbonal aspect, with the dorsal valve above (Coll. Prof. King). Ven- 

 tral aspect (Coll. Prof. Morris). A, adductor; R, cardinal; P, pedicle; V, vascular; '\ 

 O, ovarian impressions. 



i 

 Rhynchonella.. Fischer. * 



Si/n. Hypothyris, Phil. Hemithyris (psittacea) D'Orb. Acanthothyris « 

 (spinosa) D'Orb. Cyclothyris (latissima) M'Coy. Trigonella (part) Fischen 

 (not L. nor Da Costa). 



Types, R. acuta, PI. XV. fig. 18 : furcillata, fig. 19 : spinosa, fig. 20 : acu-i 

 minata, fig. 140 : nigricans, fig. 138 ; psittacea, fig. 139 (p. 8, fig. 3). 



Ske/l trigonal, acutely beaked, usually plaited ; dorsal valve elevated in K 

 front, depressed at the sides; ventral valve flattened, or hollowed along the* 

 centre, hinge plates supporting two slender curved lameUae ; dental plates i 

 diverging. 



The foramen is at first only an angular notch in the hinge-line of the ven- i 

 tral valve, but the growth of the deltidium usually renders it complete in the I 

 adult shell ; in the cretaceous species it is tubular. In R. acuminata and J 

 many other palaeozoic examples, the beak is so closely incurved as to allow < 

 no space for a pedicle. Both the recent Rhynchonella are black ; R. ocIO' i 

 plicata of the Chalk sometimes retains six dark spots. 



