Hambach — Eevision of the Blastoideae. 13 



pierced by marginal pores, as stated by C. R. Eastman. 

 They are supernumerary pieces and form a kind of accessory 

 part to the construction of the ambulacrum. It is self-evident 

 that there are differences in the size and shape of these 

 pieces, depending upon the size and shape of the lancet 

 piece as well as upon the fork piece sinus; but all have 

 undoubtedly the same physiological function, so that such dif- 

 ference in the size and shape can be of specific value only. 



THE AMBULACRAL INTEGUMENT. 



The outer surface of the ambulacrum, i. e., the lancet and 

 poral pieces, is covered by an organic and elastic integument 

 which I formerly designated as the zigzag plicated integu- 

 ment, f It gives to the ambulacral surface a striated appear- 

 ance which is not, as Etheridge and Carpenter state, ** merely 

 a delicate surface ornamentation." t This integument has a 

 di:fferent appearance in different species, and varies very much 

 in its zigzag shape and in the distance of its windings. In most 

 cases it is so eroded that it is almost impossible to make out 

 its true appearance. It may be regarded as a ribbon placed 

 obliquely on edge and running in a zigzag form forward and 

 back so that the lower opposite edges of the returning ribbon 

 unite and form the bottom of a little groove, while the other 

 or upper edges form a little ridge; see Plate II., Fig. 2 and 5. 

 These ridges show an open surface in species like Pentremites 

 jpyriformis and florealis, whereas in Pentremites sulcatus and 

 others they are firmly united to the margin of the poral pieces. 

 There is also a great variety in the distance between two 

 ridges. In Pentremites pyriformis we may count nine to ^ 

 inch, whereas in Pentremites sulcatus we have eleven to J inch, 

 etc. It begins with an acute point at the center of the del- 

 toids, surrounding two ovarian or genital openings, and does 

 this in typical species like Pentreynites sulcatus, Jlorealis , pyri- 

 yormis, etc., in such a way as to make the two openings 

 appear as one, externally. In some species, as 0. sayi, the 



* Text Book of Palaeontology, p. 192. 



t Transactions St. Louis Academy of Science, Vol. IV., p. 160. 



X Catalogue of the Blastoidea, p. 59. 



