REPTILIAN AGE. JURASSIC PERIOD. 83 



5. "Lies Ondulees," Agassiz. 



Shell intermediate in its ornamentation between the last and the next following groups ; corselet much as in 

 " Scaphoid.es ;" sides and front with longitudinal, generally smooth oostae, abruptly curved upwards 

 behind. 



Example. — T. undulata, Fbomh. (^Jurassic and Cretaceous.') 



6. "liCS Costees," Agassiz. 



Shell ornamented on the flanks and front with prominent, generally smooth longitudinal costse, with scarcely 

 visible marks of growth between. Corselet very distinct, and separated from the flanks by a well defined 

 ridge, formed usually of flattened imbricating prominences ; ornamented with numerous small tubercles, 

 or tuberculate costae, and two radiating crenulated ridges. 



Txjpe. — T. costata, Lamk. (^Jurassic and Cretaceous.') 



t. "Lea Iiissees," Agassiz. 



Shell depressed, elongate, and rounded at both extremities, smooth, or rarely with obscure traces of concentric 



costse in front ; lines of growth not strongly marked. 

 Example. — T. longa, Ag. (^Jurassic and Cretaceous.) 



8. "L,es Pectinacees," Agassiz. 



Shell short, oval subtrigonal ; without a distinct corselet ; surface ornamented with concentric radiating costse. 

 Type. — T. pectinata, Lamk. (^Existing seas.) 



These groups, it will be observed, are founded upon differences of form and orna- 

 mentation analogous to those presented in the Unionidce, where they appear to be 

 coincident with differences of structure in the animal, considered by Prof. Agassiz 

 of generic value. It does not, however, necessarily foUow from this, even if we 

 admit aU the proposed genera of Unionidce, that the sections of Trigonia under 

 consideration must be viewed as distinct genera ; since the differences of form and 

 structure observed in thfe various groups, and subordinate divisions of the animal king- 

 dom, are so infinite and varied, that we cannot always apply precisely the same rules 

 for the distinction of genera in one family, that serve to distinguish those of another. 



The genus Trigonia is closely related to Myo^lioria, of Bronn, by which it seems 

 to have been represented, during the Triassic epoch, as the latter was represented 

 during the deposition of the Permian and older rocks, by the genus Schizodus, of 

 King. For remarks on the relations of these three groups, see pages 57 and 58. 



If we exclude Myojihm-ia from the genus Trigonia, it will probably be found to 

 range back no farther than about the commencement of the Jurassic epoch, towards 

 the middle of which it seems to have nearly or quite attained its maximum de- 

 velopment.^ It was also well represented during the deposition of the Cretaceous 

 rocks, but appears to have become almost extinct at the close of that epoch, since 

 only a few doubtful instances of its occurrence in Tertiary deposits have been re- 

 corded. Some five or six species, however, are known to be still living in the 

 Australian seas. 



Tri§^onia Conradi. 



(Plate iii, Fig. 11.) 



Trigonia Conradi, Meek & Hatden, Proceed. Acad. Nat. Sci. Phila. May, 1860, 183. 

 Trigonia Conradi, Meek & Hayden, " " " " " Oct. 1860, 418. 



Shell rather small, short, subtrigonal, moderately compressed ; anterior side truncate ; base rounded ; posterior 

 side sloping obliquely from the beaks above, and apparently vertically truncate at the immediate extremity. Beaks 

 elevated, narrow, incurved, and located in advance of the middle ; posterior umbonal slopes rather distinctly 



* A few of the Saint Cassian (Triassic ?) species may possibly belong to some section of the genus 

 Trigonia. 



