ANIMALS. 71 



Attached to the hinge of both valves of Spirifer, Strovhalosia, Leptcsna, Ortkis, and 

 some other genera, is a flat part, usually termed the area, approximating more or less 

 to a position at right angles to the plane of the hinge-plate (PL XIX, fig. Qa, c ; 

 PI. XII, fig. 19 «, c ; PI. IX, fig. \a ; PL XX, figs. 6j, 7 g). It is generally deepest in 

 the perforate or corresponding valve ; the exception being in Strophomena euglyplia, and 

 a few others. In certain TerebratuUdce, such as DelthyridcBa (e. ^.pectiniformis), the area 

 is equally visible ; and there can be little doubt that the flattened space on each side 

 of the foramen in the perforate valve of some of the Jurassic Hypothyrises is a modified 

 form of the same part. 



In most of the dentigerous Palliobranchs a triangular opening intervenes between the 

 teeth of the large valve (PL IX, figs. \ b, 12 a) and the dental sockets of the other 

 (PL IX, fig. 7 b) ; this it is proposed to term the fissure,^ which is equivalent to the 

 " deltidium" of my former paper. I make this alteration to meet the views of several 

 parties who are desirous of seeing the term deltidimi restricted to the part to which 

 it has been applied by Von Buch and M. de Verneuil ; this term will therefore be used 

 in the following pages for the simple or bipartite piece often seen occupying the fissure, 

 and which I formerly named the " cicatrix." The fissure is closed in many shells by 

 the deltidium, as in Strophalosia Gerardi (PL XIX, fig. 6 b), Streptorkynchus pelargonatus 

 (PL X, fig. 24 a), and StropJiomena generally ; and open in others, for example, 

 Trigonotreta (PL IX, figs. 1 b, Via), Hypothyris psittacea, and H. excavata. It is con- 

 cealed in most Cleiothyrises, and exposed in Spiriferidias and Leptaenas ; while in certain 

 genera, it exists under both conditions ; for example, it is concealed in Pentamerus 

 galeatus, and exposed in P. conckidium ; moreover, in P. Knightii, it is exposed in a 

 young state, and closed at a later period. The fissure, with or without the deltidium, 

 does not always coexist with the area; since the latter is not associated with the 

 (open) fissure in Hypothyrises [H. psittacea), or in Cameroplioria ScJdotheimi (PL VII, 

 fig. 1 3 «) ; nor is there an area coexisting with the closed fissure in Waldlteimia Aiistrnlis 

 (PL XX, fig. 10 7). 



In TerebratulidcB and Siphonotreta generally, and in certain Strophomenas, Leptaenas, 

 and Palaeozoic Hypothyrises, the umbonal region is furnished with an aperture usually 

 of a circular form, and termed \h.Q foramen. It is apical ^\\er\ situated at the point of the 

 beak, as in Waldheimia Australis (PL XX, fig. 10 H), Epithyris siifflata (PL VII, fig. 7 a), 

 and Hypothyris marginalis, Dalm. ; sub-apical when placed below the point, as displayed 

 in most of the secondary Hypothyrises ; and sujjr a- apical y^hen situated above the point, 

 as in Leptcena analoga (PL XX, fig. 6 e). It is interrupted inferiorly by the fissure in 

 Cleiothyris pectinifera (PL X, fig. 3 a) and Terebratulacaput-serpentis, in which case it 

 may be termed an emarginate foramen ; when its outline is complete, as in Waldheimia 



' The binomial epithets, "triangular opening" (De Verneuil) and "dehidial area" (Morris), are ob- 

 jectionable, both as not being succinct enough : the latter is still movt io, a.^ one of its members {area) 

 is already current for another cardinal structure ; a circumstance liable to induce confusion. 



