352 Prof. A. Agassiz on Palmontological 



differences have existed in the shape of the spines, not only 

 of closely allied genera, but even in specimens of the same 

 species — differences which in some of the species of to-day- 

 are as great as in older geological periods. The oldest 

 Cidarida3 are remarkable for their narrow poriferous zones. 

 It is only in the Jura that they widen somewhat ; subse- 

 quently the pores become conjugated, and only later, during 

 the Cretaceous period, do we find the first traces of any 

 ornamentation of the test (Temnocidaris) so marked at the 

 present day in the genus Goniocidaris. So far, then, as 

 the Cidaridse are concerned, the modifications which take 

 place from their earliest appearance are restricted to slight 

 changes in the poriferous zone and in the ornamentation of 

 the test, accompanied with great variability in the shape of 

 the primary radioles. We must except from this statement 

 the genera Diplocidaris and Tetracidaris, to which I shall 

 refer again. The representatives of the other Triassic family 

 become extinct in the lower Tertiaries. The oldest genus 

 [Hemicidaris) undoubtedly represents the earliest deviations 

 from the true Cidaris type — modifications which affect not 

 only the poriferous zone, but the test, the actinal and the 

 abactinal systems, while, from the extent of these minor 

 changes, we can trace out the gradual development of some 

 of the characteristics in families of the regular Echini 

 now living. The genus Hemicidaris may be considered a 

 Cidaris in which the poriferous zone is narrow and undu- 

 lating, in which the granules of the ambulacral system have 

 become minute tubercles in the upper portion of the zone 

 and small primary tubercles in its actinal region, in which 

 many of the interambulacral granules become small secon- 

 daries, in which the plates of the actinal system have become 

 reduced in number and the apical system has become a 

 narrow ring, and finally in which the primary radioles no 

 longer assume the fantastic shapes so common among the 

 Cidaridee. 



We can trace in this genus the origin of the modifications 

 of the poriferous zone, leading us, on the one side, through 

 genera with merely undulating lines of pores to more or less 

 distinct confluent arcs of pores formed round the primary 

 ambulacral tubercles, and, on the other, to the formation of 

 open arcs of three or more pairs of pores. The first type 

 culminates at the present day with the Arbaciadas, the other 

 with the Diadem atidse, Triplechinidre, and Echinometradae. 

 This specialization very early takes place, for already in the 

 lower Jura Stomechinus has assumed the principal charac- 

 teristics of the Triplechinidae of to-day. 



