XVIII TESTICARDINES : EXTERNAL CHARACTERS 497 



II. TESTICARDINES 



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 Rliyiiclionella 

 (Fig. 327), or bilobed, as in Pygojoe (^= Terehratula diphyci). 

 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 (^P rohoscidella) . 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 

 Productidae are usually furnished with tubular spines, which 

 are sometimes 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 

 versa ; 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, i) 

 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 (^Stringocephalus^ Fig. 326), or tripartite, or even 

 quadripartite ; but in Rhynchonella and some other genera it is 

 rudimentary. 



VUL. HI 2K 



