130 TecJinicalities of Fossil SJiellk. 



Fig. 5 is the end of a dorsal valve of the same species, the ventral oein^.- 

 removed to shew some of the internal appendages. The two projecting points- 

 are supposed to have been the supports of the fleshy arms which constitute 

 tlie distinguishing feature in the organization of the Brachiopoda. The 

 arms were long, slender, fleshy, string-like appendages, fringed with rows of. 

 hair-like tentacula, and used by the animal for capturing its food. These 

 supports of the arms are longer in Orthis tricenaria than in any other species 

 of this genus we have seen. They can only be observed, however, in well 

 preserved specimens. They are sometimes called cardinal teeth, or dental 

 lamincB. They might he termed brachial processes. Between these is seen, 

 a third small triangular projection. This is situated in the fissure of the 

 dorsal valve, and is called the rostral tooth, or boss. It is not however a 

 tooth in the sense in which that term is used in Conchology, but simply a. 

 shelly process or projection to which a muscle was attached for the purpose 

 of opening the valves. This is the opinion of the most modern authors,. 



Fis:. 6. 



Fiff. 7. 



Figs. 6 and 7 shew the interiors of the dorsal and ventral valves of.' 

 another species of Orthis. In these the cardinal areas, triangular foramen, 

 cardinal teeth, rostral tooth, &c., may be recognised. 



Fig. 6 is the dorsal valve, and it has near its upper portion four oval 

 scars or depressions two on each side. The letter d points to the upper- 

 most on the right. The other is immediately below it. These two and the 

 two on the other side corresponding to them are muscular impressions. They 

 mark the positions of the bases of four muscules which were fastened in these 

 pits and extended to the other valve. Their office w^as to close the valves,, 

 hence they are called adductors. In the ventral value. Fig. 7, two long, 

 oval scars are also to be seen. These are the " Cardinal muscular impres- 

 sions." The muscles mserted in these were attached at the other extremity 

 to the " carcZ/ri a/ process, or ^oof /i," in the fissure of the dorsal valve, and: 

 serve to open the shell. The branched root-like marking in the dorsal valve- 

 are the *' pallial" impressions. 



It is not often that specimens can be prociu-ed wliich will exhibit all 

 these various portions of the internal structure of the Brachiopoda, and it i* 

 not necessary therefore to proceed further at present with their examination-. 

 Sufficient has been pointed out for the general purposes of tliis work.. 



The genera are distinguished not only by their outward form, but als<> 



