ECHINOTHURID^. 



79 



From the great length of the genital plates in old and large speciinen.s of 

 Phorniosoma we find from three to three and a half interamhtilacral plate.s 

 butting against their sides — a feature already existing in Palaechinidie. It is 

 well seen in a specimen of OUgoporus missouriensis Jack, from Webb City, Mo., 

 where there are three and two and a half interambulacral plates in contact 

 with the genitals, and no less than four in a specimen of Lepidechinm imbrica- 

 tus Hall, from Burlington, Iowa. An odd triangular plate is also barely in 

 contact with the outer edge of the genital plate. Between the sixth and 

 seventh plates of the two abactinal rows of interambulacral plates two 

 small rhomboidal plates are intercalated, the youngest plates of the third 



ra h 



9 mill. 



Fig. 131. P. placenta. 



row of interambulacral plates appearing near the abactinal system, much 

 as they do in the abactinal interambulacral region, where its plates separate 

 the bivium from the trivium. 



One cannot fail to be struck with the Bothriocidarid structure of the 

 actinal system of young Phormosoma (PI. 43, figs, o, 5), in which the inter- 

 ambulacral plates are excluded from the actinostomc, Figs. 131, 132, as in 

 Bothriocidaris,^ if we look upon the second and third plates of the anibu- 

 lacral areas, which form a closed ring, as still a part of the coronal plates, a 

 condition of things not uncommon in certain Clypeastroids and Spatangoids. 



I cannot understand Gregory's^ statement that in Asthenosoma the 

 apical system is reduced to ten rudimentary plates of no functional impor- 

 tance, or are altogether absent. 



The genital ring of Bothriocidaris as well as of the PaLT?chinidne is 

 much like that of the recent Echinids, only in the former the ocular plates 

 are far larger than the small plates corresponding to the genitals. 



1 Schmidt, F., Mem. Acad. St. Petersburg, XXI, No. 11 (1874). 



2 Tlie Echiuoidea, p. 294. In Bather, Gregory and Goodrich, The Echiuoderma. li)00. 



