REPTILIAN AGE. JURASSIC PERIOD. 89 



We are aware these divisions are not equally distinct, the Ifacrodontmce and 

 Arcince being more nearly related through CucuUcea, Sca])hula, Sec, than either 

 of these groups is to the Axinince. StiU, we find the Jurassic group Macrodon 

 shading off so gradually through Orammatodon, Dolahra, Cypricardites, &c., to 

 Vamixemia, and other Palaeozoic groups — some of which depart so widely from the 

 recent Arks as to be scarcely recognized as belonging to the same family — that 

 these forms seem to stand together as a distinct subfamily. At a first glance it 

 might be thought the genus Cticidlcea should be included in the Macrodontince ; 

 but on a closer inspection, it will be observed that in that genus the hinge always 

 differs from these older groups, in never having the anterior hinge plates ranging 

 obliquely forward and upwards, and at the same time the posterior ranging parallel 

 to the hinge line. Again, the posterior hinge plates are never so disproportionally 

 elongated in Gucidlcea as in the older groups, while it also differs in having small 

 vertical, mesial plates or denticles between the posterior and anterior divisions. At 

 the same time that these differences exist between the genus Oucidlcea, and all the 

 allied genera included in the subfamily Macrodontince, CucidlcEa is found to be con- 

 nected by such an unbroken series, through various fossil and recent forms, with 

 the typical Ai-cince, that it seems unnatural to separate it and the most nearly allied 

 genera into a distinct subfamily. 



Subfamily MACRODONTII^^. 

 Genus GUAMMATODON, Meek & Hatden. 



Si/non. — CucuUsea (sp.), Qoenstedt, Der Jura, 1856. 



Area ( CucuUsea) sp. Meek & Hayden, Proceed. Acad. Nat. Sci. Phila. March, 1858, 51. 



Grammatodon, Meek & Hatden, ib. Oct. 1858, 419. 

 Etym. — j-fo^ftn, a line ; o'Jiv, a tooth, in allusion to the linear teeth. 

 Type. — Area (^Cueullxa) inornata. Meek & Hayden. 



Animal unknovra. Shell longitudinally oblong, or rhombic oval, nearly or quite 

 equivalve, inequilateral ; valves rather gibbous, and without crenulated margins ; 

 umbones somewhat depressed, incurved, and not very widely separated. Liga- 

 ment area rather narrow; hinge straight, provided in each valve with a few 

 elongated, linear posterior cardinal plates, arranged parallel to the hinge margin ; 

 and a greater number of shorter, oblique anterior plates in front of the beaks. 

 PaUial line obscure ; muscular impressions faintly marked, and without a projecting 

 lamina or ridge. Surface nearly smooth, or with obscure radiating costse or striae. 



The species upon which we propose to found this genus agrees almost exactly, in 

 the character of its hinge, with Macrodon of Lycett ; but is proportionally much 

 shorter, and differs in having its pallial margin smooth, and closed, instead of crenu- 

 lated and gaping. Its beaks are also located farther back, while its muscular im- 

 pressions differ in being destitute of any ridge or prominence, such as we see in 

 Macrodon and Cucidlcea. Possibly it may be only a subgenus under Macrodon. 



It is but necessary, we think, to compare the hinge of such forms as these with 



12 September, 1864. 



