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PROFESSOR WYVILLE THOMSON ON THE ECHINOIDEA OF THE 
family ; but, if so, all the important anatomical characters, the overlapping of the plates 
of the perisom, the absence of branchial notches, the heterogeneous tube feet and their 
occurrence on the peristomial membrane up to the edge of the mouth, are omitted in 
the description. 
The genus Astropyga, rightly placed among the Diadematidse, certainly approaches 
the Echinothuridse in many respects, and resembles them closely in general appearance. 
The plates of the perisom in Astropyga overlap slightly, but they all overlap in one 
direction, towards the apical pole ; so that the arrangement in the two forms, though 
presenting a certain resemblance, is essentially different. The peristomial membrane 
in Astropyga is studded with irregular calcareous granules, with ten perforated bosses 
for the five pairs of large mouth-tentacles; and the edge of the peristome is deeply 
notched for the external branchiae. The groove on the inner surface of the teeth 
becomes gradually filled up towards the tip. The spines are solid. The pairs of pores 
are trigeminate, arranged in symmetrical arcs as in the typical Diadematidae. There is 
a marked difference in character between the apical and the oral surfaces of the test, 
which recalls this peculiarity in Phormosoma ; and the structure of the small pedicellarise 
is somewhat similar to that of the pedicellarise in the Echinothuridse. It appears, there- 
fore, that while some characters would seem to indicate a tendency to a passage from 
the Diadematidse to the Echinothuridse through such forms as Astropyga, the resem- 
blances are for the most part superficial, and very important anatomical characters 
maintain, according to our present knowledge, a broad line of distinction between the 
families. 
Phormosoma , gen. nov. 
Plates of the corona only slightly overlapping, and forming a continuous shell without 
membranous interspaces. Ambulacral and interambulacral areas of the apical surface 
of the test with irregular rows of primary tubercles with small areolse. Oral surface of 
the test different in character from the apical, with the areolse of the primary spines 
large and deep, occupying a large portion of the surface both of the interambulacral 
and of the ambulacral plates. 
Phormosoma placenta, sp. nov. (Plate LXII. and Plate LXIII. figs. 1-8.) 
The test is extremely thin, round, and much depressed. The only example which 
was procured in a condition sufficiently perfect for measurement is 80 millims. in 
diameter by about 18 millims. in height. The interambulacral areas are 32 millims. in 
width at the edge of the disk-like body, and the ambulacral areas 16 millims., exactly 
half. The peristome is 30 millims. in diameter, nearly circular, without notches, and 
the periproct 30 millims. from the outer edge of an ovarial group of granules to the 
outer edge of the opposite ocular plate. 
The apical surface of the corona (Plate LXII. fig. 2) is slightly arched, not much more 
so apparently than is necessary for the accommodation of the masticatory pyramid. 
At the circumference the test is angular, almost carinated, and the ventral surface of the 
