ANIMALS. 69 
(Pl. VIII, fig. 10a), Streptorhynchus pelargonatus (Pl. X, fig. 284), Orthis eaima,! 
and Strigocephalus Burtini (Pl. XIX, fig. 16). They are also characteristic of Pen- 
tamerus, and that curious group of shells hereafter noticed under the name of 
Lsorhynchus. 
The socket-ridges, at their junction with the hinge, are often seen approximating 
towards each other: in some cases they remain separated, as in the Crag Waldheimia 
variabilis ; but in general they conjoin, and form either a horizontal plate, as in 
Cleiothyris pectinifera (Pl. X, fig. 9 a), and Camerophoria multiplicata (Pl. VIL, fig. 5 c), 
or a concave one, as in Waldheimia Australis (Pl. XX, fig. 11 DP), and Epithytris elongata 
(Pl. VI, fig. 45 4). Besides another use, hereafter to be noticed, which this plate evidently 
serves, it forms the base of what are called the “crura of the loop” (Owen); it may 
therefore be termed the crural base. 
The crura of the loop are two slender processes, which start from the crural base, 
and project into the cavity of the shell, becoming attached. in Zerebratulide, to a 
slender, recurved or simply-folded, testaceous apparatus, known under the name of 
“the loop” (Pl. VI, fig. 45e; Pl. XX, fig. 12); and, in Spiriferide, to a pair of 
spirally-folded appendages.” The two long, slender processes springing from the 
crural base in Strigocephalus Burtimi (Pl. XIX, fig. 1c), and curving up to the ventral 
median plate, may safely be considered as the crura. These processes, however, are 
not always thus attached; since they are free in Hypothyride, as exhibited in the 
illustrative figures of Camerophoria (Pl. VIII, figs. 3, 4.7). 
Professor Owen, from his observations on certain Palliobranchs, was led to conclude 
that the crura supported the visceral parts of the animal, and that the loop and the 
spirals afforded attachment to a pair of ciliated appendages originating from each side 
of its mouth; but in Zerebratula caput-serpentis the loop chiefly supports the viscera, 
masmuch as the oral or labial appendages are, for the most part, free, projecting in 
this condition, and recurving considerably in front of the loop. In Hypothyris psittacea, 
which has neither a loop nor spirals, the labial processes are also free except at their 
origin. 
The loop is variously modified im Terebratulidias. In the typical genus, 7erebratula 
(caput-serpentis), and in Gryphus (vitreus), it is somewhat ring-shaped, and simply 
attached to the crura; in Waldheimia (e. g. Australis, vide Pl. XX, fig. 11 ) it is 
similarly attached, and with its free end bent back to nearly the crura; in Lpzthyris 
(vide Pl. VI, fig. 45 e) it is attached in the same way, but its anterior part is only 
slightly recurved ; in Zerebratella (Chilensis) and Megerlia (dorsata), it is jomed both to 
the crura and, at the anterior end, toa plate running along the medio-longitudinal line of 
! Geology of Russia, vol. ii, pl. xi, fig. 2d, e. 
2 Professor M‘Coy, in his ‘Synopsis of the Characters of the Carboniferous Limestone Fossils of 
Ireland,’ p. 127, fig. 15; and Mr. T. Davidson, in the ‘London Geological Journal,’ vol. i, have given the 
best illustrations of the spiral coils of Spiriferid@ : they are represented, but without being attached to the 
crura, in Pl. X, fig. 10, and PI. IX, fig. 8. 
