NO. 2 BRACHIOPOD SUPERFAMILY STENOSCISMATACEA — GRANT 15 



rior of the brachial valve. The trough-shaped crest also originates in 

 the apex of the valve and, in some species and some individuals, inter- 

 sects the floor of the valve there. However, it normally begins slightly 

 above the valve floor and increases in height anteriorly so that the 

 trough begins slightly anterior to the beginning of the septum itself. 



Ontogeny. — Kozlowski (1929) demonstrated that the trough cap- 

 ping the septum to form the camarophorium is an expansion of the 

 septum, not a separate structure imposed upon it. Serial sections of 

 several specimens of Devonian species of Atribonium (fig. 5; pi. 3, 

 fig. IE) show that the camarophorium begins as a small swelling on 

 each side of the crest of the median septum, at or very near the apex 

 of the brachial valve. Sections in which the duplex character of the 

 septum is obscured show the trough as a small bulb at the top of the 

 septum ; more favorable sections show it to be an expansion of each 

 side of the septum. Its rounded cross-section flattens anteriorly, and 

 about 1 mm. from the beak it is a platform with a nearly flat cross- 

 section (fig. 5c). Farther forward the platform becomes concave to- 

 ward the pedicle valve, and from there nearly to its anterior termina- 

 tion it becomes more deeply concave. Near the anterior end it shallows 

 rapidly, terminating between one-fourth and one-half the shell length 

 in different species. The trough of the camarophorium extends slightly 

 beyond the anterior edge of the septum ; the edge of the septum nor- 

 mally is concave forward, with slight extension as a ridge along the 

 floor of the valve and along the base of the trough. 



Origin. — If the adductor muscles attached within the camarophorium 

 (see discussion of musculature), that structure functioned in effect as 

 an elevated floor of the valve. An elevated or detached valve floor that 

 serves as a muscle attachment is not peculiar to the Stenoscismatacea, 

 but is found in either valve of several other groups of brachiopods. 

 Genera with the floor detached in the pedicle valve are Parenteletes 

 King (1931) among the Dalmanellacea and Orthotetella King (1931) 

 among the Orthotetacea. Mystrophora Kayser has a muscle platform 

 in the brachial valve as in the Stenoscismatacea. 



The camarophorium is not simply a detached and elevated area of 

 the valve floor, despite its probable function as a muscle attachment. 

 If it originated by progressive elevation of the muscle area, the fibers 

 of its septum would be expected to be continuous with those of the 

 valve floor. Instead, the septum of the camarophorium wedges into the 

 floor quite discontinuously, and only secondary shelly matter that 

 thickens both floor and septum is continuous from one to the other 

 (e.g., in Camerisma, fig. 12) . The camarophorium grew anteriorly from 

 the cardinal region beneath the hinge plate, and may have usurped the 

 muscle carrying function from the floor. Possibly a septum duplex 



