154 CATALOGUE OF THE BLASTOIDEA. 



The last reference we have to make in the structural history of Pentremites is the 

 discovery by Mr. G. Hambach l of a fine canal piercing the lancet-plate itself, 

 apparently in addition to that just mentioned. This canal was subsequently 

 observed by ourselves 2 , and we described it as passing inwards through the ambu- 

 lacral opening discovered by Wachsmuth and Springer to join an oral ring which we 

 regarded as belonging to the water-vascular system. Hambach has since seen this 

 ring, but has altogether misunderstood its relations, as we have explained on 

 pp. 51-54. 



Remarks. The foregoing generic description has been drawn up with the view of 

 limiting the name Pentremites to those Blastoids which resemble P. Godoni, 

 Uefrance, P. sulcatus, Roemer, and P. piriformis, Say, in their structure and 

 general appearance. The general form is ovoid or pyriform, and the ambulacra 

 are broad and petaloid. In most (if not in all) species the side plates merely rest 

 against the edges of the lancet-plates, without covering any part of them. 



Another and an equally important distinguishing feature of Pentremites is the 

 structure of the spiracles. The central end of each deltoid plate is flat and laterally 

 expanded, with a more or less marked oral ridge in the middle line that divides it 

 into two lateral halves. Each half forms the floor of a passage leading along the 

 lower part of the radial sinus over the upper ends of the hydrospire-slits. This is 

 converted into a canal by the side plates, which are wedged in between the lancet- 

 plate and the side of the radial sinus. Those nearest the centre may either meet 

 one another over the oral ridge (PI. I. fig. 4) or abut against its sides (PI. I. 

 figs. 5, 10, 11), thus enclosing the opening of the spiracle. In all those Pentremites 

 in which we have been able to examine the internal organs, the hydrospire-tubes 

 beneath the ambulacra extend along the entire length of the radial sinuses and 

 communicate with the exterior by the marginal pores and the spiracles ; but a 

 peculiar modification occurs in some species, the distal portion of the hydrospires 

 being received into the substance of the radial plates (PI. II. fig. 31), as we have 

 described on p. 95. 



Some valuable observations have been recently made by Messrs. Wachsmuth and 

 Springer, and by Mr. Hambach, on the structure of the ambulacra in Blastoids, more 

 especially as regards the lancet-plates. The former authors have described and 

 figured the lancet-plate of Pentremites as imperforate, but as resting on an under 

 lancet-plate which encloses a canal. Hambach, on the other hand, describes the 

 lancet-plate of typical Pentremites (P.Jlorealis, P. sulcatus, P. pyriformis, &c.) as 

 " pierced through the centre, in its whole length, by a very fine canal," and we are 

 inclined to think that he is right (PI. XVIII. figs. 3, 4, G). For, although we agree 

 with Wachsmuth and Springer in finding two subambulacral pieces in Pentremitis, 



1 Trans. St. Louis Ac-ad. Sci. l"-^ 1 ', vol. iv. no. 1, p. 149. 

 -' Ann. & Mag. Nat. Ili^t. L882, vol. a. p. 218. 



