152 ROMER’S DESCRIPTION OF THE CHALK 
islands which were at that time formed by central Germany and 
France. At the same time, the Cranie, which are most closely 
allied to them, exhibit their greatest development; and the 
Terebratule never again occur in like profusion. The plicated 
species of the latter genus became extinct in the upper chalk 
formation ; and those found in the older chalk differ from those 
of the oolitic group in the folds on the back (am Buckel) partly 
dichotomising. The Dichotome, which in earlier deposits occur 
but sparingly, here become very numerous; and such forms as 
Terebratula gracilis and T. pulchella are quite as peculiar to the 
chalk as the species allied to the 7. carnea; T. biplicata of the 
chalk it has hitherto been impossible to distinguish with certainty 
either from the Jurassic or from the living form. The Thecidee 
are likewise here more developed than in other periods; but they 
are by no means equal to the Terebratule in the variety and 
abundance of their forms. 
With respect to the Conchifere, the Dimyaria are proportion- 
ately more numerous than in older rocks, but still they are ex- 
ceeded by the Monomyaria. Of Oysters, the older strata espe- 
cially still contain species with longish shells, acutely dentated 
on the perpendicular margin, as they (first) occur in the Jura. 
The Exogyre occur in very large forms, and then die out. 
Among the Pectinites the natural group of the Neithee is re- 
stricted to the chalk, but occurs in nearly all its beds. Spondylus 
and Jnoceramus present a great abundance of forms; the latter 
is highly developed, especially in the gray chalk, and becomes 
extinct with the chalk formation. Avicula contains the newest 
longitudinally ribbed species. Pectunculus has already several 
large species, which call to mind those of the tertiary deposits. 
Asiartes are very rare. The Cardie approach, by some species 
which are perpendicularly truncated in front, to several living 
forms. Thetis appears to be confined to the older chalk deposit. 
Tellina, Donax, Teredo, Teredina and Fistulana here exhibit, for 
the first time, decided representatives. Goniomya occurs for the 
last time in the upper chalk. Pholadomya is likewise poorer in 
species than in the Jura, and presents no peculiar forms. 
Among the Gasteropoda (Schnecken), forms which call to mind 
Auricula are frequent. Rostellaria contains several large species. 
Pyrula and Voluta occur for the first time in the upper chalk. 
One species of Nerinea still occurs in the lower greensand, and 
there terminates its brief existence. On the other hand, we tind 
