UKI'TILIAN AGK J U II A SSIC PER IO D. ; 



.V " I.OH OiiduleoH," AGASSIS. 



Shell liitermedinto in tU ornamentation between the last and the next following group* ; conelet much as In 

 " Scuphoidet ;" sides and front with longitudinal, generally smooth ooeUe, abruptly curved upwards 

 behind. 

 /.'rum/./.. /'. uHtlulala, Putin >i. (Juraaic and Crttactout.) 



;. " !.. < ltlTs," AOAMII. 



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

 If 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 tnbercnlate costs, and two radiating crennlated ridges. 

 Type. T. eoilala, LAHK. (Jurauic and Crttaceoiu.) 



T. " l.-s Liageea," AOASSIZ. 



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



costn In front ; lines of growth not strongly marked. 

 Example. T. longa, Ao. (Jurauic and Cretaceous.) 



H. u I-es Pectinacees," AOASSIX. 



Shell short, oval snbtrigonal ; without a distinct conelet ; surface ornamented with concentric radiating cost*. 

 Type. T. peelinala, LAHK. (Existing teat.) 



Those groups, it will be observed, arc founded upon differences of form and orna- 

 mental i<m analogous to those presented in the Unionida>, whore they appear to be 

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

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

 admit all the proposed genera of l'ni>niiil(v, that the sections of Trigonia under 

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

 structure obsen rd in tin various groups, and subordinate divisions of the animal king- 

 dom, an- M> 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 Myoplioria, 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 Sr/t!z<xlu*, of 

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



If we exclude Myoplioria 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. 1 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. 



Triffonia Conradi. 



(PLATE ill, Fig. 11.) 



Trigonia Conradi, MRKK & HATDKK, Proceed. Acad. Nat. Sci. Phila. Hay, 1860, 183. 

 Triyonia Conradi, MEEK & HATDKX, " " " " Oct. 1860, 418. 



Shell rather small, short, snbtrigonal, moderately compressed ; anterior side truncate ; base ronnded ; posterior 

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

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



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

 Trigonia. 



