304 M. YOKOYAMA. 



wliich ]30steriorly meets with the nearly straight, obhqnely ascending, 

 si])hona] margin almost at a riglit angle, the corner being rounded. 

 Hinge-margin obliqnelv sleeping on the ])osterior side, and going o\ er 

 to the siplional margin without forming aiiv marked angle at the point 

 of junction. Area and escutclieon not distinctly separated, forming 

 one, more or less flat, surface whirli is slightly depressed along its 

 median liiie. Tlie other |>orti<ui of the shell, which makes an anHe of 

 about 90^ with tlie areal surf ice, is marked otf from the latter l)v a 

 rounded edge, running from the beak to tlie postero-ventral corner 

 and slopes to the anterior and ventral margins with a sliglit convexity. 

 The entire surface of the shell is smooth, except near tlie lieak where 

 n few coarse shallow coiicentric sulci are mostly f^urid. 



The shell seems to have been moderately thick. The median 

 depression of the posterior surfice is more marked in the adult tlian 

 in the younger specimens. The interrial casts show two ftrong, 

 transversely striated, diveroino* teeth. 



Among the Cretaceous Trigonia? there is none Avhich can be 

 compjired to this species. lUit in the Lias there is one, Trigomn 

 Liiifjoitcnsis Dum. (T.ycett, Monogr. i^f lîrit. Foss. Trigonia, Xo. 8, 

 p. 98, pi. X.Xn, Fig. 1-4), which shows a close affinity to it. The 

 latter, however, has a little In-oader shell and tlie posterior side 

 distinctly separated into area and escutcheon by a sharp ridge. 



Trigonia KiJcnchiana, like Tr. Lingoîicnsis, is one of those forms of 

 Trigonia which externidly exhibit a i;reat resemblance to the older 

 genus MijopJioria. 



It or-curs almost always as casts, and also often much distorted. 

 The internal mould drawn on the ])late Iras the back acciden- 

 tally depressed. Fig. 4 is from the largest specimen we got. 

 This was a liroken one, but has beeii restored iri our figure. Its 

 shape somewhat differs from that of Fig. 5, especially in its anterior 



