ANIMALS. a 
Attached to the hinge of both valves of Sparifer, Strovhalosia, Leptena, Orthis, 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. 6a, c: 
Pl. XII, fig. 19 a,c; Pl. IX, fig. la; Pl. XX, figs. 67, 7g). It is generally deepest in 
the perforate or corresponding valve; the exception being in Strophomena euglypha, and 
a few others. In certain 7erebratulide, such as Delthyridea (e.g. 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. 1 6, 12 a) and the dental sockets of the other 
(Pl. IX, fig. 7 4); 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 de/fzdium restricted to the part to which 
it has been applied by Von Buch and M. de Verneuil; this term will therefore be used 
im 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 6), Streptorhynchus pelargonatus 
(Pl. X, fig. 24a), and Strophomene generally; and open in others, for example, 
Trigonotreta (Pl. IX, figs. 1 6, 12-a), Hypothyris psittacea, and H. excavata. It is con- 
cealed in most Cleiothyrises, and erposed in Spiriferidias and Leptzenas ; while in certain 
genera, it exists under both conditions ; for example, it is concealed in Pentamerus 
galeatus, and exposed in P. conchidium; moreover, in P. Knightii, it 1s 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 (//. psittacea), or in Camerophoria Schlothermi (Pi. Vil, 
fic. 13 a); nor is there an area coexisting with the closed fissure in Waldheimia Australis 
(Pl. XX, fig. 10 1). 
In Terebratulide and Siphonotreta generally, and in certain Strophomenas, Lepteenas, 
and Paleozoic Hypothyrises, the umbonal region is furnished with an aperture usually 
of acircular form, and termed the foramen. It is apical when situated at the point of the 
beak, as in Waldheimia Australis (Pl. XX, fig. 10 AH), Hpithyris suffata (Pl. VU, fig. 7 a), 
and Hypothyris marginalis, Dalm. ; sub-apical when placed below the point, as displayed 
in most of the secondary Hypothyrises ; and swpra-apical when situated above the point, 
as in Leptena analoga (Pl. XX, fig. 6e). It is interrupted inferiorly by the fissure in 
Cleiothyris pectinifera (Pl. X, fig. 3a) and Terebratula caput-serpentis, in which case it 
may be termed an emarginate foramen ; when its outline is complete, as in Waldheinia 
1 The binomial epithets, “triangular opening’? (De Verneuil) and ‘“‘deltidial area” (Morris), are ob- 
jectionable, both as not being succinct enough: the latteris still more so, as one of its members (areca) 
is already current for another cardinal structure ; a circumstance liable to induce confusion. 
