184 THE CANADIAN NATURALIST. [June 



plates, which uuderlie that portion of the ambulacrum of 

 Pentacrinus that runs from the mouth to the base of the arm. 

 These resemble the lancet plates, in their being azygos and not 

 connected "with pinnulce; but then, on the other hand, they differ 

 from them in having, a portion at least, of the respiratory tubes 

 on their ventral aspect. Mr. Rofe says that, " in many species of 

 Pentremite, if not in all, this lancet plate is in reality a com- 

 pound plate, formed of two contiguous plates, extending from the 

 bottom to the to the top, and then turning right and left round 

 the summit-openings, they pass down the adjoining sinus to 

 form half its lancet-plate, leaving at the apex of the body a pen- 

 tagonal aperture supposed to be the mouth. In some weathered 

 specimens, the two parts of the lancet plate are separate; and in 

 many they appear to meet only at the top and bottom of the 

 cross section, leaving a lozenge-shaped opening between them." 

 (Geol. Mag., vol ii, p. 249.) In a large specimen of P. ohesus 

 (Lyon and Cassiday) which was given to me by Mr. Lyon, a 

 polished section shows that one of the lancet plates is thus 

 divided, but in general no trace of a suture can be seen in these 

 plates. 



There are several points in the structre of the ambulacra of 

 Pentremites that are well worthy of the study of those who have 

 plenty of well preserved specimens. Among these, I would 

 direct special attention to the markings in the ambulacrum of P. 

 piriformis.. The median groove, which I suppose to have been 

 exclusively occupied by the ovarian tubes, sends off branches 

 right and left alternately, towards the sides of the ambulacrum. 

 These branches do not run directly to the ambulacral pores. 

 Each of them terminates at a point between the inner extremities 

 of two of the pores. There is at this point a small pit which 

 appears to be the socket of an appendage quite distinct from the 

 pinnule. The groove docs not reach the socket of the pinnule, 

 which is situated further out, between two of the pores. On the 

 other hand a small groove runs from each pore inward, and termi- 

 nates at another socket, about half-way between the pore 

 and the main median groove of the ambulacrum. It would thus 

 appear that besides the ordinary pinnules, there were two other 

 rows of appendages on each side of the median groove. 



The general conclusions at which I have arrived from the 

 above, are, that all the principal plates that compose the shell 

 of Pentremites^ belong to the perisomatic system of Professor 



