CRINOIDEA AND ECHINOIDEA 



29 



little over .40; and tlie largest, 24 mm. has a peristome only 8 

 mm. or .33. Large specimens have the peristome often under 

 .30 of their horizontal diameter. 



Another interesting growth change is in the plating of the 

 periproct. In the smallest specimen, the suranal plate covers 

 the periproct almost completely, jnst as it does in adult Geno- 

 cidaris maculata. In specimens a little larger, there are three 

 other periproctal plates; the four plates have the relative size 

 and position of those in adult Trigonocidaris alhida, though the 

 suranal may be relatively larger. In a specimen 8 mm. in 

 diameter, there are eight periproctal plates, but the suranal is 

 much the largest. The number of plates then increases and the 

 relative size of the suranal decreases until, in a specimen 17 

 mm. in diameter, there are 14 plates and the suranal covers 

 only about one-fourth of the periproct. In the adult (43 mm. 

 in diameter), there are about twenty plates and except for its 

 position the suranal is scarcely distinguishable. 



The specimens in the Iowa collection were taken at the fol- 

 lowing places : 



Station 57. Pourtales Plateau, 24° 18' N. x 81° 18' W., 200- 



225 fms. 29 specimens. 

 Station 58. Pourtales Plateau, 24° 19' N. x 81° 19' W., about 



200-225 fms. 1 specimen. 

 Station 64. Florida: off American Shoal Light, 110 fms. 1 



specimen. 

 Locality unknown. 11 specimens. 



Lytechinus euerces 



H. L. Qark, 1912. Mem. M. C. Z., 34, p. 247; pi. 107, figs. 4-6. 



It is interesting to find half a dozen specimens of this spe- 

 cies in the Iowa collection but it is most unfortunate that there 

 is no locality label, so there is no clue to the depth at which 

 they were taken. As they were mixed with Trigonocidaris 

 alhida, EcJiirius gracilis and Genocidaris maculata, it is fairly 

 certain that they were collected with those three species on 

 Pourtales Plateau. They are all young, the test-diameter being 

 only 10-15 mm. For the differences between this species and 

 those with which it occurs see below (p. 33) under Genocidaris. 



