UNDULATE. C5 



adult forms Lave the few lower costse of the anteal series more or less wrinkled and 

 obscure, these take the direction of the lines of growth, and therefore curve upwards to 

 the anterior border ; other examples have the whole of the anteal series forming smooth, 

 irregular and unequal, oblique, wrinkled costse, but in all instances this series occupies 

 only the smaller portion of the costated surface, their junction with the other series is 

 always anteal to the middle of the costated surface ; the posteal series are fewer, much 

 larger, and more regular; they form prominent nodose ridges, which descend almost 

 perpendicularly from the carina, enlarging downwards, and forming acute angles with the 

 anteal series ; about a moiety of the posteal series attain the lower border : our largest 

 specimen has twelve of these costae. Few examples of the genus have the lines of growth 

 so strongly marked as in the adult examples of T. literata, they impress the costse very 

 conspicuously. 



Young specimens from nine to twelve lines across the valves are remarkable for the 

 prominence, delicacy, and beauty of their ornamentation ; they are slightly more lengthened, 

 than the adult form, but are less oblong and quadrate than the little Trigonia pulchella, 

 of Agassiz, to which their ornamentation approximates considerably. 



Dimensions of a large specimen. 



Length . . . . . .2^ inches. 



Opposite measurement .... 1-j^ „ 



Diameter through the united valves . , . 1^ „ 



Several Jurassic species approximate to T. literata in the general plan of their 

 ornamentation. Lyrodon literatum, Goldf., a larger species, has the general figure more 

 lengthened, and the area much larger in proportion. T. suhglohosa^ Mor. and Lye, is 

 distinguished by the more globose form, by the large tuberculated carinal, and by the few 

 very large posteal varices. T. Painei, Lye, on the other hand is much more depressed, 

 and the costse fewer. T. V-costata, Lye, has not the two series of costae broken 

 and separated as in the present form, it is also much less convex : other of the Undulata 

 are more remotely allied to it. 



History, StratiyrapJdcal position, and Locality. Messrs. Young and Bird, in their Geolo- 

 gical Survey of the Coast of Yorkshire (1822), first described T. literata : they assigned it 

 to the Lower Lias Shale, but without any locality : the figure of it given upon plate viii of 

 their work is very indifferently executed. In the year 1829, Professor Phillips, in his 

 ' Illustrations of the Geology of Yorkshire,' gave a much better figure : he assigned the 

 species to the Lower Lias Shale of Robin Hood's Bay, and figured it with the fossils of 

 that stage ; he also noticed its occurrence in Upper Lias Shale upon the authority of Mr. 

 Williamson (p. 161). In 1850, D'Orbigny in his 'Prodrome,' also erroneously placed it 

 (printed T. lyrata) in his !6tage Sinemurien, and gave the vicinity of Metz (Moselle) as a 

 locality. In addition to T. literata, Agassiz placed the five following species in the 



