BRITISH FOSSILS. 



■5 



surface. The odd or posterior interambulacral area has the plates con- 

 siderably distorted by the position of the vent. Those above it increase 

 gradually, and are about six or seven in number. From four to six 

 bound the sides and the lower margin of the vent, and three pair fill up 

 the sub- anal region as far below as the base of the post-oral spinous 

 space, which is composed of a pair of large triangular plates with 

 curved outer sides. The number and arrangement of the interambu- 

 lacral plates does not seem to differ much in any of the varieties, but 

 the number of the ambulacrals differs in every specimen which is not 

 exactly similar to some other. In a cast of the form cor-testudinarium 

 the number of pairs of pores in the anterior ambulacrum was (petaloid 

 region) 12 + (plane region) 15 ; in the antero-laterals (petaloid region) 

 25 + (plane region) 15 ; in the postero-laterals 19 + (plane region) 16 

 + (ventral region) 5. In a cast of the gibbus form, the respective 

 numbers ranged in the same order were — 



Anterior ambulacrum . . 22+12 

 Antero-lateral ditto . . . 35 + 19 

 Postero-lateral ditto . . . 30 + 17 + 5. 



Flint-casts often show very clearly the arangement of the oviducal 

 pores and eyes. The number of pairs of pores increases with age. 



In the following table I have contrasted the dimensions and numbers 

 of pair of pores in the dorsal ambulacra of twelve adult specimens taken 

 from among all the chief varieties. It will be seen that these characters 

 are very variable, and rMt sufficient to warrant specific distinction. 



Variety, 

 and Number of Specimen 

 in 



Survey Collection. 



Length. 



Breadth. 



Height. 



Number 

 of Pores 

 in a row 

 in each 

 Antero- 

 lateral 

 Avenue. 



Number 

 of Pores 

 in a row 

 in each 

 Postero- 

 lateral 

 Avenue. 



Number 

 of Pores 

 in a row 

 in the 

 Anterior 

 Ambula- 

 crum. 



1. y. Extreme form, " Gib-l 

 bus" of Lamarck . j 



Inches. 

 3 



Inches. 

 3 



Inches. 

 3 



46 



37 



28 



2. y. Extreme but small) 

 form . . . .J 



2^ 



^ 12 





43 



30 



22 







2 





36 



34 



25 



4. y. " Gibbus" of Goldfuss 





2t^ 





36 



31 









1^ 





33 



25 



18 



6. Inter y and /3, nearer y . 



2 



2^2 





31 



21 



19 



7. Inter a. and 7 . , . 



Mi 



2 



1 -3 

 ^ 15 



29 



21 



20 



8. a. Tumid form . • 



2 



2; 



ll^ 



27 



22 



20 



9. a. Depressed .... 





2^ 





32 



28 



20 





2^ 



2f2 





34 



24 



20 



11. iS. Eostrated .... 



2tI 



^ T2 



U 



34 



25 



18 





0 



2 



1t^5 



31 



22 



22 



