Morphology of the Blastoidea. 283 



that the covering consists only of fragments of broken-up 

 pinnulas, which were washed into the arabulacral furrows and 

 remained there." This is quite possible; but it is no argu- 

 ment whatever against the existence of these covering-plates 

 in other specimens in the form of a double series of minute 

 alternating pieces, as described by Meek and Worthen*. 

 The covering is not often preserved, bat I have seen it in a 

 few individuals of Granatocrinus Norwoodi and of Oropho- 

 crtnus stelliformis, and best of all in a fine example of Pen- 

 tremif.es sulcatus kindly lent me by Mr. Waehsmuth. In 

 this specimen, which is more than usually well preserved, 

 there are no openings on the summit at all, for it is covered 

 by a large number of plates in an irregular group. Minute 

 pentagonal plates gradually differentiate themselves in the 

 re-entering radial angles of this covering, and extend down- 

 wards on to the ambulacra in an alternating double row, 

 which roofs in the food-groove as completely as do the 

 covering-plates on a pinnule of the recent Pentacrinus aster ia 

 or of any Palseocrinoid. The form of the plates is so regular, 

 and the manner in which their apices interlock is so very 

 marked, that there can be no question about their being in 

 their natural position, and not of merely accidental occurrence. 

 I cannot therefore agree with Mr. Hambach in the inference 

 which he draws from the specimens which he has examined, 

 with respect to the unimportance of these plates. 



Mr. Hambach's exposition of his views respecting the 

 morphology of the Blastoidea is supplemented by " a few 

 words about the proposed new classification of Mr. Car- 

 penter. He says, ' The basis of the classification which we 

 have been led to adopt is the morphology of the hydrospires 

 and of their external openings, the so-called spiracles,' etc. 

 The general rule which governs the classification of our fossil 

 Echinodermata is the difference in the number and compo- 

 sition of pieces forming the exoskeleton. But, contrary to 

 this rule, Mr. Carpenter considers the hydrospires as very 

 characteristic and of much systematic value, although I 

 believe he agrees with Billings in considering them respi- 

 ratory organs." f 



In the expressive language of Mr. Hambach the 

 word " etc/' stands for the following sentence : — " We find 

 that the structure and distribution of these organs, together 

 with the arrangement of the various elements composing 



* Proc. Philad. Acad. 1869, p. 85. 



f Trans. St. Louis Acad. vol. iv. p. 543. 



