NO. 1229. CAMBRIAN BRACHIOPODA—WALCOTT. 693 



The cast of the visceral cavitj^ in the ventral valve shows it to have 

 been relatively small and usually confined to the posterior half of the 

 shell, although when the shell is laterally compressed it may ))c drawn 

 out to the center of the valve, as in the specimen illustrated ])y Dr. 

 G. F. Matthew. There are no traces of a median septum in the \'entral 

 valve; in the dorsal valve a slightly elevated median line occurs at the 

 bottom of the groove between the central uuiscle scars, that extends 

 forward to the anterior margin of the visceral cavity ])eyond the 

 anterior lateral muscle scars. The visceral cavit}' of the ventral valve 

 extends forward to about the center; in some shells it is back of the 

 center, and in others a little in front. It A-aries in width and outline 

 very much as the shells vary, being wide in broad shells and narrow 

 in elongate forms. 



The markings left on the shell by the vascular sj'stem are very strong, 

 and beautifully preserved in some portions. In some shells there is a 

 double groove with a slight ridge between; in others the rido-e is larg-e, 

 onh' a trace of an outer groove remaining; in some young shells the 

 grove is broad and shallow; in all shells the large size of the main 

 vessels is shown by the broad, strong grooves or ridges left on the 

 shell. It frequently happens that the deeplv indented lines of pits on 

 the lines of growth deeply indent the grooves and rounded ridges left 

 by the main vessels and mark them off into sections. The interior 

 and lateral vessels left narrow but strong grooves or ridges on the shell, 

 which, however, are usually obscured by the strong pitting of the 

 surface. The parietal scar surrounds the visceral cavity in each valve, 

 crosses the course of the main vascular vessels, and comes back around 

 the spaces occupied by the muscle scars, terminating at the edge of 

 the area at the flexure in the ventral valve; termination unknown on 

 the dorsal valve. 



Some of the muscle scars are finely shown in the dorsal valve and 

 fairly well in the ventral. The luubonal scar of the ventral valve is 

 divided, the pedicle scar l)eing situated between the two parts. In the 

 dorsal valve the umbonal scar is close to the area and extends nearl}^ 

 as far each side of the median line as the length of the area. 



The scars of the central muscles in the ventral valve are crowded in 

 with the middle and outside laterals within the trapezoidal space. In 

 the dorsal valve the}" are located on a low ridge each side of a central, 

 longitvidinai median depression; they are_ elongate oval in outline, 

 their major axis being subparallel to the median line of the shell; line 

 longitudinal lines cross the scars in the best preserved specimens; the 

 ridge on which the central scars occur varies in strength, but it 

 appears to be present in all adult shells; it narrows gradualh' poste- 

 riorly and rather rapidlj^ to the inner side of the anterior lateral muscle 

 .scars. The anterior laterals of the ventral valve are placed well ])ack 

 on the narrow space between the edge of the area and the main Aascu- 



