Morphology of the Blastoidea. 293 



a membranaceous integument "*. Even if we admit, for the 

 sake of argument, that such is really the case, there are other 

 possible explanations of the fact than that put forward by 

 Mr. Hambach. 



In my former criticism of this doctrine I made the follow- 

 ing- remark f : — " It is certainly a very singular phenomenon 

 that the 'collapsed tentacles ' of Pentremites obliquatus and 

 of P. crenulatus respectively should have assumed shapes 

 in limestone which are so very different, but yet constant 

 for those particular species." I purposely selected these two 

 types, because good figures of their ambulacra are given by 

 Romer f, and must therefore be tolerably well known ; but I 

 might readily have chosen two other species which would 



Transverse section of a restored ambulacra! field : — a, tentacle ; b, hydro- 

 spiric sac ; c, integument covering ambulacral field ; d, poral piece ; 

 <», duct beneath tbe lancet piece ; /, lancet piece ; g, canal perforating 

 the same ; s, nervous (?) canal. 



The above is an exact copy of Mr. Ilambach's fig. 1, together with 

 his explanation of it. It ^ill be seen that there is no s in the 

 cut, although this letter appears in the explanation below it. On 

 the same page (p. 538), however, where he is speaking of the canal 

 within the'lancet piece (g of his figure), he says, " As these canals 

 are perforating entirely solid calcareous substance and apparently in 

 no direct communication with the hydrospiric sac, I suppose that 

 they served for the reception of the nervous system. '' It has been 

 suggested by Mr. R. Etheridge, Jun., and myself (' Annals,' April 

 1882, p. 218) that these canals lodged the radial water-vessels; but 

 Ave do not believe that the latter had any lateral tentacular branches. 



present a still greater difference in the characters of their 

 supplemental pore-plates. The bearing of these facts on Mr. 

 Hambach's theory is self-evident ; but he has not attempted 

 to explain them, although he has had an opportunity of doing 

 so. On the contrary, he has published a figure, which I re- 

 produce, showing a transverse section of a restored ambulacral 

 field. The tentacles are here represented as communicating 



* Trans. St. Louis Acad. vol. iv. p. 152. 



t ' Annals,' ser. 5, vol. viii. pp. 423, 424. 



% Op. cit. Taf. iii. fig. 116, and Taf. iv. fig. 15 d. 



Ann. & Mag. N. Hist. Ser. 5. Vol. xv. 21 



