from the Inferior Oolite of the Cotteswolds. 227 



the Costatse, the Clavellatae, the Qiiadratse, the Scabrae, and the 

 Glabrae. The Costatce have a figure more arched than the other 

 sectional forms ; they have smooth regular longitudinal ribs, 

 which are separated from the posterior slope or area by a carina 

 more or less prominent, but which, with advance of age, often 

 becomes nearly obliterated ; this is the marginal carina ; the area 

 has transverse striations which are frequently decussated by lon- 

 gitudinal plications, and by one or two, more prominent than the 

 others, that which bounds the area posteriorly bemg the inner^ 

 carina ; should a third carina be present between the two others, 

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

 ment is always plicated or reticiilatcd. 



The Costatce 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 

 ClavellatcE accompany the Costatce in their stratigraphical distri- 

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

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

 costse divided into tubercles, serrations, or irregular varices which 

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

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

 the Goniomijee ; 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 differences 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 nev^er large ; the 

 lanceolate post-ligamental space is smooth, except in certain 

 subcretaceous forms, which have the space strongly costated, as 

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

 these are the T. sulcataria. 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 sm'face 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- 

 berculatcd 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 ; their 

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

 crescentic rather than trigonal; the oblique costse are elevated 



16* 



