ANIMALS. 69 



(PI. VIII, fig. 10 a), Streptorliynchus pelargonatus (PL X, fig. 28^), Ortliis eximia,^ 

 and Strigoceplialus Burtini (PL XIX, fig. 1 b). They are also characteristic of Pen- 

 tamerus, and that curious group of shells hereafter noticed under the name of 

 Isorhynchus. 



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 WaldJieimia 

 variabilis; but in general they conjoin, and form either a horizontal plate, as in 

 Cleiothyris 2iectinifera (PL X, fig. 9 a), and Cameroplioria multiplicata (PL VIII, fig. b c), 

 or a concave one, as in Waldheimia Australis (PL XX, fig. 1 1 D), and Epithytris elonyata 

 (PL VI, fig. 45 b). 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 TerebratulidcB, to a 

 slender, recurved or simply-folded, testaceous apparatus-, known under the name of 

 "the loop' (PL VI, fig. 45 e; PL XX, fig. \1F); and, in Sjnriferida, to a pair of 

 spirally-folded appendages.^ The two long, slender processes springing from the 

 crural base in Striyocephalus Burtini (PL XIX, fig. 1 c), 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 Hyjjothyrida, as exhibited in tlie 

 illustrative figures of Cameroplioria (PL VIII, figs. 3, 4y). 



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 Terebratula caput-serpentis the loop chiefly supports the viscera, 

 inasmuch 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 in Terebratulidias. In the typical genus, Terebratula 

 {caput-serpentis) ,' QXid in Gryphus (vitreus), it is somewhat ring-shaped, and simply 

 attached to the crura ; in JFaldheimia (e. g. Australis, vide PL XX, fig. 1 1 F) it is 

 similarly attached, and with its free end bent back to nearly the crura ; in Epitliyris 

 (vide PL VI, fig. 45 e) it is attached in the same way, but its anterior part is only 

 slightly recurved ; in Terebratella {Chilensis) and Meyerlia {dorsata), it is joined both to 

 the crura and, at the anterior end, to a plate running along the medio-longitudinal line of 



^ Geology of Russia, vol. ii, pi. xi, fig. 2 c?, e. 



2 Professor M'Coy, in liis '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 Spiriferida : they are represented, but without being attached to the 

 crura, in PL X, fig. 10, and PL IX, fig. 8. 



