88 PALAEONTOLOGY OF THE UPPER MISSOURI. 



linear plates. Ligament external, attached to a more or less developed 

 cardinal area ; cartilage occupying a series of small marginal pits (usu- 

 ally leaving linear grooves in the area as the shell advances in its growth), 

 or very rarely collected within a single larger central cavity. Muscular 

 impressions two ; pallial line simple. 



Animal without siphons or true palpi ; mantle margins open, simple 

 or fringed, often provided with ocelli ; labia formed of the extremities 

 of the branchiae ; gills oblique or pendent, separated behind, or united 

 to a membranaceous septum ; foot large, bent, generally grooved, and 

 with plain or crimped margins. 



As was first observed by Dr. Gray, the hinge in this and some allied families, 

 although in most cases apparently provided with a numerous series of small 

 teeth, has really but a posterior and an anterior tooth, which are divided verti- 

 cally, obliquely, or horizontally into small, more or less numerous interlocking 

 plates. These divisions, Dr. Gray thinks, are analogous to the transverse ridges 

 produced by the striae or furrows in the teeth of Trigonia. In the typical Arks 

 (that is, viewing A. Noce as the type), and some of the other genera, the divisions 

 of the teeth cut the hinge margin nearly or quite at right angles, but they are 

 found to become more and more oblique, as we pass from group to group, until in 

 Cucullcea, Macrodon, &c., they range, particularly behind the beaks, parallel to the 

 cardinal margin. 



The family Arcidce, including the various fossil and recent genera, seems to 

 embrace three, and possibly four, more or less marked subfamilies, distinguished 

 mainly by the arrangement of the cardinal plates, and partly by the general form 

 and obliquity of the shell, &c. These subfamilies may be arranged and character- 

 ized as follows : 



t. Arcinae. 



Shell more or less oblong, or subrhombic ; nmbonal axis oblique, hinge margin straight or more or less 



arched ; cardinal plates crossing the hinge margin at various angles, or rarely dividing it horizontally 



near each extremity. 

 Includes Area, Barbatia, Striarca, Anadara, Senilia, Lunarca, Argina, Noetia, Litharca, ParaUelepipedttm, 



Scaphula, Cucullxa, and probably Isoarca f and several undefined fossil genera. (Paleozoic 1 to modern seas. ) 

 3. Macrodontinae. 



Form, hinge line, and umbonal axis, generally much as in the Arcinse. Anterior hinge plates crossing the 



cardinal margin obliquely forward and upward ; posterior plates ranging parallel to the hinge line, often 



long and linear ; mesial plates obsolete. 

 Includes Macrodon, Grammatodon, Cypricarditis, and probably Vanuxemia,' Megalomus, Megambonia, Dolabra, 



and some undefined genera. (Palaeozoic to Jurassic.) 



3. Axininae. 



(a.) Shell orbicular, or suborbicular ; cartilage as in Arcinx; umbonal axis nearly vertical ; hinge line regu- 

 larly arched ; cardinal plates short, and arranged as if radiating from an imaginary point below the hinge. 

 Includes Axinsea. 



(6.) Form and hinge as in subsection (a.). Cartilage occupying a single pit at the middle of the hinge. 

 Includes Limopsis. 



1 If Megalomus, Hall, 1852, Vanuxemia, Billings, 1858, and Megambonia, Hall, 1859, are, as is 

 thought to be the case, all synonymous, then the rules of priority would compel us to adopt the name 

 Megalomus for the group. Until the relations of these proposed groups have been more clearly de- 

 termined, it is probably better to retain them all, provisionally, as distinct genera. 



