BRACHIOPODA. 655 



careous, with unequal valves ; the beaks usually prominent, or some- 

 times obtusely rounded, and either massive and solid, or divided by 

 a partition into two chambers. There is a well-developed hinge-area 

 (fig. 503), and a wide deltidium, bounded by two ridges, the inner 

 ends of which serve as teeth, though true teeth are not present. 

 Each valve is furnished with muscular platforms, which, in the 

 typical species, are elevated and doubly vaulted. The principal, or 

 only, genera of the family are Trimerella, Dinobo/us, and Mono- 

 merella, and the family is confined entirely to the Ordovician and 

 Silurian rocks. The differences between the above-mentioned genera 

 depend mostly upon internal characters, which can hardly be made 

 clear except by an examination of actual specimens, and as these 

 very generally occur in the form of internal casts, the study of the 

 forms of this group is attended with exceptional difficulties. 



Order II. Articulata 

 (=Ctiste7iterata, Arthropomata, Apygid). 



In this order of the Brachiopods the valves of the shell are hinged, 

 usually by means of teeth and sockets ; the lobes of the mantle are 

 not completely free ; and the intestine ends blindly. This order 

 includes the families of the Productidce, Strop homenidcz, Koninck- 

 inidce, Spiriferidce, Atrypidtz, RhynchonellidcE, Terebratididce, Stringo- 

 cephalidce, and Thecidiidce, of which only the Rhynchonellidce, Te?-e- 

 bratulidce, and Thecidiidce are represented by living forms. 



Family i. Production. — In this family the shell is entirely free, 

 or is attached to submarine objects by the substance of the ventral 

 valve or by means of spines. The valves may be regularly articulated, 

 or may be kept in place by muscular action alone. There is a 

 straight hinge-line, and the outer surface is more or less largely fur- 

 nished with tubular spines, while the inner surface exhibits numerous 

 funnel-shaped punctures. The arms are not provided with calcified 

 supports, but there are well-marked muscular and vascular impres- 

 sions. The ventral valve (fig. 505, ve) is convex, while the dorsal 

 valve (do) is concave, and is furnished with a prominent cardinal 

 process (ca), to which the divaricator muscles are attached. The 

 interior of the dorsal valve exhibits a pair of large central dendritic 

 adductor-impressions, separated by a median ridge (a a), a pair of 

 reniform vascular impressions (/), and two shelly prominences (/), 

 which probably served to support the bases of the arms. The 

 interior of the ventral valve exhibits a pair of large dendritic ad- 

 ductor-impressions, situated close beneath the incurved beak, a pair 

 of large lateral striated divaricator-impressions (d d), and two anterior 

 depressions (s) which probably lodged the spiral arms. 



