403 



Plicatula spinosa. A variety in its young state with a smooth 

 test, has been described by E. von Eoemer under the name of 

 Ostrea orbicularis (Ool. p. 61, T. 3, fig. 14) ; it is very likely the 

 same as Rolle's and Goldfttss's Plicatula sarcinula, which I 

 have entered in the general list of fossils from the Spinatus Zone. 



Unicardium Janthe, d'OEB (Prod. 8, 179). 



A form similar to this, found in the Spinatus Beds, occurs as 

 low down as the Zone of Am. Jamesoni, and throughout the 

 Middle Lias infrequently. It appears to be the shell meant by 

 d'ORBiGNT under the scanty description he gives " Espece 

 voisine de I' Unicardium cardioides mats subequilaterale, ovale, 

 arrondie aux extremites : it is probably only a varietal form of 

 U. cardioides. 



The inconstancy of character discerned in the Ostrece, creates 

 a difficulty, which, apart from speculation or discussion of 

 biological causes, can only be practically met by seizing upon 

 well marked forms, to which geologists have agreed by tacit 

 consent to attach names. Such a well understood form is that 

 of Ostrea cymbium, which exists in the Spinatus and other Zones 

 of Middle Lias, and was adopted as a classical appelation of the 

 Middle Lias by Leymkie in 1872, who designates the Middle 

 Lias the Cymbium stage of the Liassic system — the Charmouthien 

 stage of Karl Mater, 1874. The Ostrea cymbium of our 

 Spinatus Zone is generally of small size. 



Avicula calva, n. sp. U. Schlonbach. A small tumid species 

 I refer to this name ; the shell is not unlike A. papyracea, but 

 more tumid, and has faint concentrical markings just perceptible. 



Modiola omata (Moore). (On the Middle and Upper Lias of 

 S. W. England P. vii, fig. 7.) Mr. Moorb obtains this shell 

 from Ilminster. "Shell transversely ovate; gibbous, beaks 

 small, incurved, nearly anterior ; pedal region short, rounded ; 

 dorsal and posterior margins rounded, middle and ventrical 

 margins slightly sinuate ; back of the shell diagonally gibbous, 

 and covered by striae, which pass from the umbo to the dorsal 

 margin and posterior end. A slight sulcus di\ddes the gibbous 

 back from the anterior end on which the strise are obsolete. 



