4 



BRITISH FOSSILS. 



the ambulacral segments, as the mouth is of the interambulacral. I 

 have not met with the spines. The shell is thin. 



The largest example I have seen measured eight-twelfths of an inch 

 in diameter. The proportion of elevation to breadth varies in almost 

 every individual, and is quite as variable as it is well known to be in 

 the recent Echinus. The specimen mentioned was five-twelfths of an 

 inch in height, and had 34 plates in vertical succession, composing each 

 half-segment, and consequently there were above 100 pairs of pores in 

 each avenue, indicating as many suckers. 



In four examples from the same locality the proportion of diameter 

 to height was respectively as 17 to 10, 15 to 11, 12 to 11, and 12 to 8. 

 The Arhacia conica of Agassiz, of which we have no other description 

 than that it is near granulosa, " Mais plus haute et plus conique," 

 cannot, therefore, be regarded as a species, or even as a marked variety. 

 With the variations of proportion the shell becomes more or less 

 globular. 



Locality and Geological Position. — Very abundant in the upper 

 green-sand of Warminster. (Survey Collection.) Chute Farm, Wilts. 



On the Continent it was first noticed in the upper green-sand of 

 Regensburg, in Bavaria. It occurs in France in the Craie chloritee of 

 ITsle d'Aix, Le Mans, and (^Arhac. conica) Calvados. 



The Echinus pelos, from the Neocomian of the canton of Neuchatel, 

 is its nearest ally and predecessor. 



M. Grateloup states that it is found at Dax, and in the white chalk of 

 Pouillon. 



Explanation of the Plate. 



Fig. 1. A specimen of ordinary dimensions, seen from above. Fig. 2. The same from 

 the side; and Fig. 3, from below. Figs. 4, 5, and 6. Outlines of specimens of different 

 ages, showing extreme variations of form. Fig. 5 appears to be the Arhacia conica of 

 Agassiz. Figs. 7 and 8. Upper and under magnified views, showing the arrangement and 

 articulations of the plates, which, for better display, are partly represented without their 

 tubercles. Fig. 9. The circle of ovarian and ocular plates. Fig. 10. Ambulacral and 

 interambulacral plates, with portions of avenues taken from the central portion of the 

 side, showing the arrangements of the primary and secondary tubercles. Fig. 11. Portion 

 of avenues near the mouth, showing that the apparent single arrangement of the pairs 

 of pores is deceptive, and that they are really three-ranked. Fig. 12. One of the primary 

 tubercles, seated on its elevated base, and surrounded by secondary granules. 



E. Forbes. 



April, 1849. 



