226 



MANUAL OF THE MOLLUSCA. 



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

 provided with spiral processes ; muscular impressions grouped as in Terebra 

 tula ; vascular" impressions consisting of two principal trunks in each valvi 

 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 setse. 



Ven- 



Ventral aspect. Umbonal aspect. 



Fig. 140. Rh. acuminata, internal casts. 



Fig. 140! Umbonal aspect, with the dorsal valve above (Coll. Prof. King), 

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

 O, ovarian impressions. 



RHYNCHONELLA.. Fischer. 



Syn. Hypothyris, Phil. Hemithyris (psittacea) D'Orb. Acanthothyris 

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

 (not L. nor Da Costa). 



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

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



Shell trigonal, acutely beaked, usually plaited ; dorsal valve elevated in 

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

 centre, hinge plates supporting two slender curved lamellae ; dental plates 

 diverging. 



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

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

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

 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. ocfo 

 plicata of the Chalk sometimes retains six dark spots. 



