FROM THE WHITE CHALK. 333 



slightly prominent under lip, and there radiates from its circumference a stellate dis- 

 position of large tubercles (fig. 1 h, e), which highly ornament the oral opening. The 

 small vent opens at the infra-marginal region near the termination of the posterior carina 

 and behind an elevation and thickening of the test, which may be described as the anal 

 area ; the periprocte is oval in the longitudinal direction (fig. a, b). 



Afmities and Differences. — The abundance of this species in the Upper Chalk beds 

 causes many variations of form to be displayed which have been described and 

 figured by collectors as distinct species; I shall mention some of the most remarkable. 

 The upper surface is often inflated and gibbous, very regularly convex, angular, and a 

 little contracted towards the base; this is the Anmicltytes gihha, Lamarck, the Ananchyteii 

 striatus (var. sudylobosus), Goldfuss (' Petref. GermanicE,' pi. xliv, fig. 3). Sometimes this 

 variety has the body more elongated, the upper surface less gibbous and marked in the 

 posterior region, with the carina more developed, the base always angular at the circum- 

 ference and slightly contracted ; this is the Echinocorys scidalns, Leske, and AnancJiytes 

 striata, Lamarck. Sometimes the upper surface, whilst preserving a gibbous and convex 

 aspect, is elevated and conoidal ; this is the Ananchytes conoideus, Goldfuss (pi. xliv, 

 fig. 2), the variety -y of Professor Forbes. When the ambitus is considerably contracted, 

 and the upper surface inflated, the shell has a remarkable subspherical appearance ; it 

 then forms the Ananchytes Gravesii, Desor. This variety sometimes attains a large size, 

 then the upper surface loses its gibbous aspect and becomes altogether conical, it is then 

 the EcJiinocorys pustulosus, Leske, and the Ananchytes conica, Agassiz. 



All these varieties, and several others which are mentioned in the works of different 

 authors, such as Ananchytes carinatus, Defrance, A. pyramidatus. Port., A. semiglohus, 

 Lamk., A. crassissima, Agass., A. Eudesii, Sorignet, &c., appear to be modifications of 

 the same type, and merely varieties of the fine formed shell I have figured as Echinocorys 

 vulgaris, Breynius. 



On this subject Professor Forbes observes,^ " To this last view I feel compelled to 

 assent, for, however distinct the forms termed ovata, gihba, striata, seniiylobus, conica, &c., 

 may seem when selected and contrasted in the cabinet, every one of them is linked with 

 the others by the most delicate shades of gradation. Neither in degree of elevation, 

 rotundity, flatness of base or curve of back, smoothness or roughness, can I find any 

 constant character. Scarcely two individuals out of more than a hundred examples now 

 before me exhibit the same proportions." 



The Echinocorytes ovatus, Leske (PI. LXXVn,fig. 9), is a well-marked form found in 

 the Chalk of Meudon. When compared with the Echinocorys vulgaris (PL LXXVH, 

 fig. 1 a, b, c), its general outline is seen to be more ovoid, elongated, and more sensibly 

 rostrated posteriorly ; its upper surface is thicker and more inflated anteriorly ; the ambitus 

 is roimder, less angular, and the under surface more depressed around the mouth-opening ; 

 the ambulacra are larger as they approach the summit, and form a less acute angle ; the 



^ ' Memoirs of the Geological Survey ; British Organic Remains,' decade iv, pi. vi, p. 4, 1852. 



43 



