90 PALAEONTOLOGY OF THE UPPER MISSOURI. 



that of the recent Area Noce, usually regarded as the type of the genus Area, to be 

 satisfied that they cannot be included in the same group, in accordance with correct 

 rules of classification. 



The group under consideration will probably include several other Jurassic, and 

 possibly a few Cretaceous species, though we cannot, with any degree of confi- 

 dence, attempt to define its exact geological range. 





 Grammatodon inornatus. 



(PLATE III, Fig. 9, 9o, 96.) 



Area (CuctiWara) inornate, MEEK & HAYDEN, Proceed. Acad. Nat. Sci. Phila. March, 1858, 51. 



Grammatodon inornatus, MEEK & HAYDEN, Ib. Oct. 1862, 419. 



Comp. Cucullxa Munsteri (ZEITEH), QDENSTEDT, Der Jura, tab. 13, fig. 38 ; tab. 18, fig. 34 ; tab. 23, fig. 8. 



Shell subrhomboidal, or oblong-oval in outline, rather gibbous in the nmbonal region. Anterior end rounding 

 up from below, and forming an angle of about ninety degrees with the hinge line above ; posterior side a little 

 broader than the other, obliquely truncate above, and somewhat narrowly rounded below ; base nearly straight 

 along the middle, but not exactly parallel to the hinge line, rounding up more gradually in front than behind. 

 Beaks rising somewhat above the hinge, incurved, and very slightly oblique, located a little in advance of the 

 middle ; posterior umbonal slopes prominently rounded. Hinge comparatively long, but not quite equalling the 

 greatest length of the shell ; posterior hinge-plates three or four in each valve ; anterior much shorter, and usually 

 numbering about six or seven. Ligament area not very broad. Surface apparently smooth. 



Length, C.75 inch ; height, 0.45 inch ; breadth, 0.46 inch. 



Quenstedt figures several forms similar to this under the name of Cucullfea 

 Munsteri, in his " Der Jura," though none of them appear to agree exactly with 

 our shell. The true C. Munsteri, as figured by Goldfuss (Petrefact. Germ. tab. 

 122, fig. 10), diifers, at least from our species, in having a rather distinct ridge 

 along the cardinal margin. All of our specimens also appear to be entirely desti- 

 tute of radiating striae ; though there may be very fine radiating lines, where the 

 surface is well preserved. 



Locality and position. Southwest base of Black Hills, associated with other 

 Jurassic fossils. (Type 201.) 



FAMILY UOTONIDJE. 



Shejl equivalve, inequilateral, regular, smooth, plicate or tuberculate, 

 nacreous within ; epidermis thick, covering a prismatic cellular layer ; 

 margins closed and smooth within ; ligament external. Hinge varying 

 with the genera and subordinate groups. Muscular impressions deep ; 

 pedal scars three in each valve, two behind the anterior adductor, and 

 one before the posterior ; pallial line simple. 



Animal with mantle margins disconnected, excepting between the anal 

 and branchial regions ; not produced into siphonal tubes, but fringed in 

 the branchial, and usually plain in the anal regions. Foot very thick, 

 tongue-shaped, often byssiferous in the young, but rarely so in the adult. 

 Gills elongate, subequal, free or connected with the mantle or abdominal 

 sack behind. Labial palpi usually united behind, 



Conchologists differ very widely in regard to the number of generic and sub- 

 generic heads under which the numerous species included in this family should be 



