6 



BRITISH FOSSILS. 



propriety of uniting Discoidea rotula of Agassiz, and Discoidea favrina 

 of Desor, since the character of the notching on the casts varies in dif- 

 ferent specimens. Discoidea conica, of Desor, I take to be the same 

 species, assuming a more pentagonal form, a variety apparently not 

 uncommon among British examples. The Discoidea rotula of Mr. 

 M'Coy, from the upper chalk, must surely be something quite distinct. 

 The young of the species, to which I would reserve the name Galerites 

 {Discoidea) favrinus^ might be confounded at first sight with suhuculus^ 

 but is very different on close examination. It presents an orbicular 

 outline, with a strong tendency to become pentagonal. It is regularly 

 convex above, with more swelling sides, and, ccmsequently, a flatter 

 summit than suhuculus has, whilst its margins are often nearly as much 

 rounded as in that species. The interambulacral segments are, in their 

 widest part, thrice the breadth of the ambulacrals, and the ambulacra 

 widely diverge from each other when proceeding from the summit, 

 characters which will at once distinguish between this form in its young 

 state and subuculus. 



Locality and Geological Position. — It abounds in the upper green 

 SAND of Warminster, Chute Farm, Wilts; in the junction bed of green 

 SAND and CHALK MARL at Maiden Bradley, Wilts. (Survey Collec- 

 tions.) Lower chalk, Weymouth (Morris). In the white chalk of 

 Kent, but not so common. Morris (Cat. p. 52) mentions its occurrence 

 in the lower green sand of Hythe, Kent. 



Foreign Distribution. — On the Continent it occurs in the chalk marl 

 of France and Germany. Hillsconglomerat of Essen (Miiller). 



Explanation of the Plate. 



Figs, i and 2. Upper and under views of a green-sand specimen. Fig. 3. Profile of a 

 conical green-sand specimen allied to var. /3. Fig. 4. Sub-depressed variety. Figs. 5, 6. 

 Magnified representations of the dorsal and ventral surfaces, showing the arrangements of 

 the plates. Fig. 7. Arrangement of the ovarian and ocular pores on the summit. Fig. 8. 

 Ambulacral and interambulacral plates, with their tubercles, granules, and pores. Fig. 9. 

 Primary tubercle surrounded by granules. 



E. Forbes. 



April, 1849. 



