98 



TIIKEE CRUISES OF THE " BLAKE. 



during the cretaceous and Jurassic periods. They are, with the 

 Pourtalesise, the forerunners of the true spatangoids. They have 

 many features in common with the flat clypeastroids, such as 

 their tuberculation, the character of their pedicellariae and 

 spines, and the structure of the apical system (Fig. 366), while 

 the structure of the anal system and the general facies of the 

 test rather allies them to the true spatan- 

 goids. But neither the Nucleolidse nor the 

 Pourtalesiae are possessed of fascioles, an 

 eminently spatangoid structure. These 

 specialized bands of minute spines are 

 slightly developed in some of the creta- 

 ceous genera, and their rudimentary form 

 exists to-day in such types as Hemiaster. 

 Their exact function is not yet known. 

 They take their greatest developments in 

 such modern genera as Schizaster. Some 

 light has been thrown on their development by the discovery 

 of a deep-sea species of Macropneustes, which shows a gradual 

 transition between the tuberculation of the test (Fig. 367) and 

 specialized areas corresponding to fascioles. 



M^ 



Fig. 366. — Neolampas 

 rostellata, magnified. 



Fig. 367- — Macropneustes spatangoides 



important bearing as indicating the spe- 

 cies which are likely hereafter to be pre- 

 served as fossils, and shows us how diffi- 

 cult it may become, even when we have 

 such an abundant and characteristic echi- 

 nid fauna as that of the West Indies, to 

 reconstruct it from the future fossils. 

 We may also notice that the genera of 

 which we so frequently find the dead tests 

 are the same which have been known as 



characteristic of the West Indies since 

 the earliest tertiary. We cannot expect 

 to find represented among the fossils the 

 Echinothurise, Pourtalesiae, and many of 

 the Echinida?, since after death they read- 

 ily fall to pieces, and may then be dis- 

 solved, like many species of inollusks, at 

 great depth, before they become protected 

 by a covering of deep-sea ooze. 



