of Edinburgh, Sessio7i 1871-72. 
617 
Echinothuridae was proposed, the fossil-germs Ecliinotliuria , saga¬ 
ciously described by the late S. P. Woodward, from an imperfect 
specimen from the upper chalk being taken as the type. The 
specimens procured were referred to two genera and three species. 
In the genus Phormosoma the plates of the perisom only 
slightly overlap, and fit so closely as to form a complete calcareous 
casing without any membranous fenestras. Although constructed 
essentially on the same plan, the apical and oral surfaces of the 
test differ from one another singularly in character, the oral sur¬ 
face being almost uniformly covered with large areolar depressions 
surrounding spine tubercles. 
One species, Phormosoma 'placenta , n. sp., was dredged in deep 
water off the Butt of the Lews, and some fragments were met 
with in gravel from the Rockall Channel. 
In the genus Calveria , the plates of both the amhulacral and 
inter-ambulacral areas form large expansions towards the middle 
line of the area, while the outer portions of the plates are narrow 
and strap-shaped, leaving large fenestrse filled up with membrane 
between plate and plate. The oral surface of the body does not 
differ markedly in character from the apical. 
Two species of this genus were taken, Calveria hystrix , n. sp., 
with a strong perisom, of a nearly uniform rich claret colour, from 
deep water off the Butt of the Lews ; and Calveria fenestrata , n. sp., 
more delicate, with wider spaces between the plates, the body of a 
greyish colour, rayed from the apical pole with bright chocolate. 
It is very possible that the genus Asthenosoma , described by 
Professor G-rube, may belong to this group, but the description of 
that form hitherto given is not sufficient for identification, as the 
points of structure on which the families of the Ecbinidea are dis¬ 
tinguished from one another are not noticed. With this exception, 
the form which most nearly resembles them is Astropyga , which, 
however, is in every respect, except in habit, a true Diadema , with 
the peristomial margin deeply notched for external branchiae, and 
all the other characters of the family. 
