from the Inferior Oolite of the Cotteswolds. 249 



it is the median carina ; the lanceolate space posterior to the liga- 

 ment is always plicated or reticulated. 



The Costata are remarkably prominent in the lower and 

 middle Oolite rocks ; in the upper Oolites and lower portion of 

 the Cretaceous series they diminish and almost disappear. The 

 Clavellata accompany the Costata in their stratigraphical distri- 

 bution ; in this section I would include the Clavellata, the Un- 

 dulata, and the Scaphoides of Agassiz, all of which have their 

 costae divided into tubercles, serrations, or irregular varices which 

 are disposed in rows, either concentric, oblique or excentric ; 

 sometimes they are bent to form an angle after the manner of 

 the Goniomya ; the links which connect the one with the other of 

 these features are too transitive to allow of any clear sectional 

 divisions when they are applied to a large number of species, 

 neither will the diiferences of form afford any more certain guide. 

 In the Clavellata, as in the former sections, a carina more or less 

 distinct separates the area from the tuberculatcd portion of the 

 surface ; the area is transversely striated and is never large ; the 

 lanceolate post-ligamental space is smooth, except in certain 

 subcretaceous forms, which have the space strongly costated, as 

 in the contemporaneous Scabra, to which they form a passage ; 

 these are the T. sulcataria 9 Lam., T. muricata, Goldf., and T. 

 Lusitanica, Sharpe. 



The Quadrata have the figure rendered subquadrate by the 

 largeness of the area, the upper border of which is nearly hori- 

 zontal; its surface is flattened, and sometimes constitutes the 

 larger half up the shell ; there is no marginal carina, neither is 

 there any clear line of separation between the area and the tu- 

 berculated portion of the surface ; the Quadrata are fewer than 

 the Clavellata, which they seem to replace in the Cretaceous and 

 upper portion of the Oolitic system ; Trigonia nodosa, Sow., is a 

 well-known example. The Scabra constitute a fourth and very 

 natural section, which are distinguished not less by their figure 

 than by the beauty and variety of their ornamentation; tin ir 

 figure indicates a change from the usual figure of the genus ; it is 

 crescentic rather than trigonal ; the oblique costae are elevated 

 and serrated ; they are continued across the depressed area, the 

 separating carina (marginal) being replaced only by a smooth 

 groove. The stratigraphical distribution of the Scabra is equally 

 characteristic ; they are exclusively Cretaceous, and seem to re- 

 place the Costata of the Oolitic rocks ; Trigonia alaformis and 

 spinosa are examples. The fifth section, or Glabra, are destitute 

 of costa, rows of tubercles or of carinae upon the area ; their sides 

 have large longitudinal plications, and are nearly smooth; 7W- 

 gonia gibbosa and affinis are examples : this section, of which few 

 species are known, has not occurred beneath the upper division 



