the Crinoidea, Cjstidea, and Blastoidea. 145 



the Crinoids the plnnula3 are attached to the radial joints of 

 the arm ; in Pentremites they are not connected with the lancet 

 plate, but with the pore-plates ; in P. iiyriformis they appear 

 to me to stand in sockets excavated in the suture between the 

 pore-plates proper and the supplementary pore-plates. Miller 

 compared them to the series of azygos plates which underlie 

 that 'portion of the ambulacrum oi 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 pinnulge ; 

 but then, on the other hand, they differ from them in having 

 a portion at least of the respiratory tubes on their ventral as- 

 pect. Mr. Rofc says that " in many species of Pen fre»^^Ve5, if 

 not in all, this lancet plate is in reality a compound plate, 

 formed of two contiguous plates extending from the bottom of 

 the sinus 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 penta- 

 gonal 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 structure 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 ambula- 

 crum of P. pyriformis. The median groove, which I suppose 

 to have been exclusively occupied by the ovarian tubes, sends 

 off branches, right and left alternately, toward the sides of the 

 ambulacrum. These branches do not run directly to the am- 

 bulacral 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 appen- 

 dage quite distinct from the pinnule. The groove does not 

 reach the socket of the pinnule, which is situated further out, 



also have performed the office of teutacula in conveying food to the 

 mouth, which was pei'haps provided with an exsertile proboscis ; or may 

 we not rather suppose tliat the animal fed on the minute beings that 

 abounded in the sea-water, and that it obtained them, in the manner of 

 the Ascidia, by taking them in with the water ? The residuum of diges- 

 tion appears to have been rejected through the mouth." (Journ. Acad. 

 N. S. Phil. 1825, vol. iv. p. 296.) 



