Brachioj)oda of tlie Cincinnati Group. 7 



are distinguished from the LameUibrancJdata by the absence of any 

 special branchial apparatus, the respiratory function being performed 

 by the mantle, and by the structure of the shell. 



They are all marine shells, equal sided and inequivalve. The valves, 

 instead of being placed on each side of the animal as in the LameJli- 

 branehiata, are situated above and below it ; so that they are called 

 dorsal and ventral, instead of right and left valves. The ventral valve 

 is usually larger than the dorsal and projects beyond it at the beak, 

 where it is generally perforated to allow the passage of the attaching 

 ligament. Sometimes, however, the shell is attached by the beak, 

 and in other cases, as in the Lingididce, the attaching peduncle passes 

 out between the umbones. The dorsal valve is always free and 

 imperforate. 



The valves are usually articulated by two curved teeth, springing 

 from the ventral valve and locking into corresponding cavities in the 

 dorsal valve. In Crania and some other genera having a flat ventral 

 valve, usually adherent, the hinge line is destitute of teeth. The 

 valves are opened by the "cardinal muscles," and closed by the 

 adductors. The body of the animal usually occupies only a small 

 portion of the cavity of the shell, close to the hinge. 



With the exception of the corals, more than three fourths of all 

 the fossils found in the Cincinnati Group belong to this class. 



In the preparation of this Monograph, I have used the best defini- 

 tions of the S2:)ecies accessible, and particularly the re-definitions by 

 Prof. Meek in the Ohio Paleontology. 



The following are the characters ascribed to this class by Prof. 

 McCoy : 



"This class .contains a large group of symmetrical, usually thin, 

 sharp-edged shells. The border of the mantle is little developed, and 

 the group derives its name from two long arms, one on each side of 

 the mouth, which may be regarded as an enormous development of 

 the labial palpi of LameHibranchiata, among which Anomia alone 

 shows anythuig appi'oachiug the same development. These arms are 

 hollow tubes, coiled or doubled up, and capable of a greatei" or less 

 protrusion by the contraction of a doubly oblique set of muscular 

 fibers, Avith which they are coated, acting on the fluid contained 

 within them; they unite in fi'ont of the mouth, and have their outer 

 margin fruiged with filaments, the motion of which, wdiether protruded 

 .beyond the shell or not, is supposed to produce a strong current of 

 water, conveying nutrient particles toward the mouth ; the absence in 

 them of large blood vessels shows they are not respiratory. The edge 

 of the mantle is also fringed with long, fleshy, or semi-corneous filaments, 



