26 FOSSIL ECHINI OF THE WEST INDIES. 



mm. in length, and 19 mm. in width. The large specimen mentioned 

 shows the auricles extending vertically and joined in suture over the 

 ambulacrum as usual in the genus. The species superficially resem- 

 bles Echinometra lucunter, which is abundant in the living West Indian 

 fauna, but so far has not been found fossil. It differs strongly in that 

 lucunter has pore-pairs in arcs of 7 to 8 pairs in each plate, whereas 

 prisca has only 3 or 4 pairs to a plate. Some specimens show 3 only, 

 as far as preserved, others show 3 or 4 pore-pairs, and a specimen from 

 Cuba (station 5255) shows 4 pairs to a plate throughout. Cotteau's 

 figure 22 of an ambulacrum, enlarged, shows 3 pore-pairs only to a 

 plate. The ambulacra in lucunter widen to the peristomal border, 

 whereas in prisca they become narrow at this area. These two are 

 sufficient characters to separate the two species, which at first sight 

 look alike. 



Echinometra prisca with its few pore-pairs is a very primitive species 

 in the genus, being comparable to the young of lucunter and other 

 species. Moreover, it is the only species of the genus occurring fossil, 

 as far as I am aware. The Recent species of Echinometra have from 

 4 to 8 or 9 pore-pairs to a plate, and it is most interesting that this 

 fossil species with only 3 or 4 pairs to a plate makes such an approach 

 to the next lower family, the Echinidae, which is characterized by 3 

 pore-pairs to a plate. 



The feature of the pore-pairs in the ambulacral plates of Echinometra 

 prisca, as described, is in direct accord with the important principle 

 that in a genetic series the characters in the young of a higher group 

 and also in the geologically oldest members of the higher group make 

 a close approach to the characters of the adults of a more primitive 

 group in a phylogenetic series. 



Oligocene, Anguilla formation, island of Anguilla, Guppy collection 

 ex Cleve, 8 specimens, cotypes, U. S. Nat. Mus. No. 115398. Anguilla, 

 Guppy collection, 4 specimens, U. S. Nat. Mus. No. 115414. Road 

 Bay, south side, near western end of point on north side of Bay, 

 Anguilla, T. W. Vaughan collector, 1914, 1 specimen, U. S. Geol. 

 Sur. station 6969a. Geologic age doubtful, Santiago de Cuba, 1 

 specimen, U. S. Geol. Sur. station 5255, U. S. Nat. Mus. No. 328222. 

 Cliff face, east shore of Guanica Harbor, Porto Rico, 300 feet south of 

 small cave of station 341 ; the beds are a shaly limestone, somewhat 

 stratified, 1 specimen, C. A. Reeds collector, station 375, Expedition 

 of the New York Academy of Sciences, the Porto Rican Government 

 and the American Museum of Natural History cooperating. 



