Braehiopoda of the Cincinnati Group. 19 



cavo-convex, each with a hinged area notched in the center; ventral 

 valve with prominent teeth ; muscular impressions occupying a saucer- 

 shaped cavity, with a raised margin ; adductor central ; cardinal and 

 pedicle conjoined, lateral, fan-like ; dorsal valve with a tooth-like car- 

 dinal process between two curved brachial processes ; adductor impres- 

 sions quadruple ; vascular impressions consisting of six principal trunks 

 in the dorsal valve, two in the ventral ; the external branches turned 

 outward and backward, inclosing wide ovarian spaces. Indications 

 have been observed, in several genera, of horizontally coiled, spiral 

 arms; the space between the valves is often very small. The shell 

 structure is punctate, except in a few instances, where the original 

 texture is probably obliterated. 



Some authors prefer to classify the shells of this fiimily under the 

 name Strophomenidce. It includes, with other genera, the Orthis Hemi- 

 pronites, Strophomtna, and Leptena. 



Genus Orthis — (Dalman, 1827). 



[Etym. — Orthos, straight]. 



Shell variable in shape, suborbicular or quadrate ; valves equally or 

 unequally convex ; socket valve sometimes slightly concave, with or 

 Avithout a mesial fold or sinus ; hinge line straight, generally shorter 

 than the width of the shell; both valves furnished with an area, 

 divided by a triangular, open fissure, for the passage of the pedicle 

 fibers ; beaks more or less incurved — that of the larger valve generally 

 more produced; surface smooth, striated or ornamented by simple 

 bifurcated or intercalated ribs ; structure minutely or largely punc- 

 tated; valves articulating by means of teeth and sockets. In the 

 interior of the larger, or ventral, valve the vertical dental plates form 

 the walls of the fissure, and extend from the beak to the bottom of the 

 shell ; between these a small, rounded mesial ridge divides the muscu- 

 lar scars, which extend over two elongated depressions, margined on 

 their outer side by the prolonged bases of the dental plates ; the car- 

 dinal muscles appear to have occupied the greater portion of the 

 anterior divisions of these two depressions, the pedicle muscles occu- 

 pying the external and posterior part of the same space ; the adductor 

 was probably attached to each side and close to the mesial ridge. In 

 the socket valve the fissure is partially or entirely occupied by a more 

 or less produced simple shelly process, to which were aflixed the cardi- 

 nal muscle fibers ; the inner socket walls are considerably prolonged 

 mto the cavity of the shell, under the shape of projecting laminae, to 



