TESTICARDINES : EXTERNAL CHARACTERS 497 



II.— TESTICAKDINES 



External Characters 



It is to this division that the great majority of the Brachio- 

 poda belong ; and the diversity of form, of ornamentation, and 

 of internal characters is correspondingly greater than in the 

 Ecardines. 



A transversely or longitudinally oval shape of shell is the 

 commonest ; but sometimes it is triangular, as in Rhynclionella 

 (Fig. 327), or bilobed, as in Pygope ( = Terelratula dijjhya). The 

 ventral valve is usually more convex than the dorsal, and the 

 former may be prolonged into a tube by the accelerated growth 

 and infolding of the anterior and lateral margins, producing a 

 very abnormal form (Frohoscidella). The external surface of the 

 valves is frequently ornamented with more or less prominent 

 radiating ribs ; and fine concentric growth-lines are commonly 

 shown, and may be developed into coarse ridges or wrinkles, 

 particularly in old individuals. The members of the family Fro- 

 ductidae are usually furnished with tubular spines, which are some- 

 times of great length, and served to anchor the free shells in the 

 mud, or were twisted round Crinoid stems and similar objects. 



In the ventral valve of many genera there is a median sinus, 

 with a corresponding fold in the dorsal valve, and rarely vice 

 versd ; sometimes the fold and sinus are double. 



The hinge line is either curved or straight, and the valves 

 are articulated by means of a pair of " hinge-teeth " (Fig. 329, t) 

 in the ventral valve, which fit into corresponding sockets in the 

 opposite valve. Some genera have the teeth very rudimentary, 

 or have lost them altogether. The teeth are frequently sup- 

 ported by " dental plates," and the sockets by " socket plates " 

 (e.g. Conchidium, Figs. 324, 325). A few genera with a long 

 hinge line have the whole of it denticulated (Stropheodonta). In 

 the dorsal valve medianly close under the hinge line is a shelly 

 protuberance- — the " cardinal process " — to which the diductor 

 muscles are attached. It is sometimes of great length and 

 forked {Stringocephcdus, Fig. 326), or tripartite, or even quadri- 

 partite ; but in Rhynclionella and some other genera it is rudi- 

 mentary. 



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