182 



BRACHIOPODA 



to two short, free, curved lamellae. Trias (and in the wider sense, 

 Ordovician) to present day. Ex. R. tetrahedra, Lias ; R. cynocephala 

 and R. (Acanthothyris) spinosa, Inferior Oolite; R. plicatilis, Chalk; 



A B 



Fig. 85. Rhynchonella (Pugnax) acuminata, Carboniferous Limestone. 

 Internal casts. A, Ventral valve. B, Dorsal valve and posterior 

 part of ventral. V, 'vascular' impressions ; 0, genital impressions ; 

 A, adductors; R, divaricators ; P, muscles of the peduncle. (From 

 Woodward.) Natural size. 



R. (Hemithyris) psittacea, Pliocene and Recent. Acanthothyris is a 

 sub-genus, including forms which have the shell covered with spines ; 

 chiefly Jurassic. Most of the Palaeozoic species formerly referred to 

 Rhynchonella are now regarded as belonging to distinct genera or 

 sub-genera, viz. : — Rhynchotreta, Camarotmchia, Wilsonia, Uncinulus, 

 Hypothyris, Pugnax. 



Terebratula (figs. 70, 74, 86). Shell biconvex ; oval, elongate 

 or rounded ; surface nearly always 

 smooth ; often with two folds on the 

 dorsal valve and two corresponding 

 sinuses on the ventral. Hinge-line 

 curved. Umbo of ventral valve trun- 

 cated by a circular foramen with 

 a deltidium at its base. Brachial 

 skeleton in the form of a short 

 loop extending only about a third 

 the length of the shell. Jurassic 

 to present day. Ex. T. maxillata, 

 Inferior and Great Oolite. ward -) 



Fig. 86. Terebratula (Liothyris) 

 vitrea, Becent. Interior of 

 dorsal valve, showing the bra- 

 chial skeleton. (From Wood- 



