44 A. Agassiz—Connection between the Cretaceous 
growth, while in the Zemnopleurus it never becomes entirely 
obliterated. In Temnopleurus we might say the Salenid abac- 
tinal system was more readily traced, and still better in Arbacia, 
while in both these genera the Cidarid features of large primary 
tubercles is retained in a different degree; and in Arbacia, in 
We readily trace through Pileus, Holectypus and Discoidea affir- 
ities to Galerites, and the fossil Conoclypeide, while with the 
appearance of Galerites, Fibularia, and Echinocyamus we have 
the element of the Clypeastride and Scutellide; and their 
relationship to the Cassidulide is well shown in the simple am- 
bulacral system of some of the genera, and the rudimentary 
auricles are still to be traced among the Echinolampade, while 
the affinity of the earliest Cassidulidse, Hyboclypus, Galeropygus 
and the like, to Pygaster, which culminate in our day with but 
slight modifications in Echinoneus, show how clearly related the 
earlier Spatangoids were with tue genera to which the Clypeas- 
troids are most closely related, which in their turn will show a 
most unmistakable relationship to the Desmosticha, so much so 
that it seems difficult to say whether some of the Hchinolam- 
pade of the present are not more closely related to the Galeri- 
tide, from the slight development of the pelatoid system, and 
the presence of jaws or of rudimentary auricles. 
ready, in the Jura, Pygaster shows the method of the pas 
sage of the anal system from the intevior of the anal ring to the 
odd interambulacral space, and we find genera such as Holecty- 
us and Discoidea, in which it occupies in succession all possible 
positions between the apical system and a place close to the 
actinosome ; and, the passage once effected in the Clypeastroids, 
we readily go from a mere cireular or elliptical opening place¢ 
either in the axis, or obliquely or transversely to it, to an open: 
ing in a slight groove or a more or less deep groove occupyibg 
this same odd interambulacral space, having its climax in the 
Kchiuobrissine ; and then we most naturally pass to an opening 
holding a certain relation toa more or less distinct beak which, 
combined with the subanal plastrou enclosed by the subanal 
fasciole, we can gradually trace from a simple plastron flush 
with the test, as in the earliest Holasteridz, to the Echinocal 
dize, to the Brissine, and finally to the Pourtalesiz, to a plas 
tron extending as a slight beak below the anal system, and fin- 
