CONCLUDING SYNOPTICAL OBSERVATIONS. 219 



Trias, its species occur in France, Alsace, Hanover, Brandenburg, Saxony, Swabia, 

 Wurtemberg, and the Tyrol. In Britain it is represented by a single species obtained 

 only in one bed of the highest or Rhaetic stage of the Trias, exemplified by Myophoria 

 postera, Quenst., Jura., tab. i, figs. 3 — 6, discovered by Mr. Moore at an obscure locality 

 in the county of Somerset. The anteal direction of the urabones serves effectually to 

 separate it from the Trigonice costatce, irrespective of the distinctions afforded by the 

 hinge-processes. 



The exterior ornaments in Myophoria postera, p. 215, are analogous to, but are 

 altogether distinct from, the Jurassic costata ; the opposite valves more especially ofier 

 important differences. 



Throughout the several zones of Molluscan life disclosed by the Lower Lias the 

 genus Triyonia is known only by one or two minute, imperfectly defined specimens, 

 doubtful generically, obtained by Professor Tate in the Jrmatus-zone, and named by 

 him Triyonia modesta, ' Yorkshire Lias,' p. 386, pi. xiv, fig. 4, depicted also of the 

 natural size upon our Plate XLI, fig. 13 : but in the absence of more satisfactory 

 specimens the presence of this genus in the Lower Lias remains doubtful. 



The Middle Lias of France has produced rather abundantly, and that of Britain very 

 rarely, and apparently localised in each country, a single species of the Triyonia yladra, 

 T. Linyonensis, Dum. (Plate XXII), p. 98, an abnormal form when compared Avith the 

 genus generally, almost devoid of surface ornaments, and in that feature approaching to a 

 group of the more ancient Triassic Myophoria^ but even in this exceptional species the 

 figure agrees with that of the genus subsequently developed ; the umbones, unlike those 

 of Myophoria, are recurved or directed backwards, thus departing both from the figure of 

 that genus and from the Conchifera generally, but agreeing with the Triyonioe, and 

 imparting a degree of concavity both to the area and escutcheon. T. Linyonensis is 

 limited to the main ironstone band of the Zone of Anuiionifes spinatus, both in France 

 and in Yorkshire. 



Since the notice of the discovery of this species in Yorkshire by Professor Tate in 

 1872, English geologists working upon the long course of the Middle Lias have searched 

 for this remarkable Triyonia in the midland and southern counties of England without 

 success with one exception, in which several specimens were obtained in the Spinatus-zone 

 in the vicinity of Banbury by Mr. E. A. Walford of that place during the past summer 

 of 1877 ; the same gentleman has also forwarded to me an internal mould of Triyonia 

 obtained by him in the Zone of Ammotntes Henleyi in the same vicinity ; it is not certain 

 to what species it belongs, apparently it does not difi"er materially from moulds of T. 

 .Linyonensis ; in either case it establishes the presence of the genus Trigonia in a lower 

 zone of the Middle Lias. 



In the Cleveland Ironstones this species continues to be one of its most rare forms, 

 notwithstanding that the bed has been extensively worked and its fossils diligently 

 collected at numerous localities over wide areas and during many years. 



