1904.] Affinities of Palceodiscus and Agelacrinus. 39 



We have seen that Billings demanded pores through the series of 

 ambulacral plates in E. Bigsbyi, and Bather has established this fact, as 

 also that there is present an uppermost series of covering plates. This 

 demolishes Jaekel's suggestion (8, p. 45) that the plates through which 

 the pores pass might be covering plates. Whether these pores serve for 

 the passage of ampullae or podia is an open question and we propose to 

 discuss it in the later portions of this paper. Hall claims to have 

 discovered pores in Agelacrinus. Various authors have, however, 

 with regard both to Agelacrinus and other genera, given accounts 

 which directly contradict the views expressed above. 



Roemer, in 1851, obtained a cast of Agelacrinus rheanus which led 

 him to surmise that the ambulacral grove was occupied by single 

 flooring plates. 



F. B. Meek, 1873, who described a peculiarly preserved specimen of 

 [. cincinnatiensis says, " The shell (of the Brachiopod upon which it 

 was sessile) had separated in such a manner as to take with it the 

 under side of the Agelacrinites, and leave its upper side in the matrix 



situated as to expose its inner surface. The inner side of each arm 

 >r ray is here seen to be composed of a single series of quadrangular 

 >ieces that are not imbricating." 



Hemicystis latiuscula, described by Jaekel (8, p. 17) agrees in structure 

 dth the above. " Die Einnen werden hier deutlich unterlagert von 

 jiner einfache Reihe querverlangerter etwas winklig vorgezogener 

 'lattchen." 



None of these authors discovered in these genera any trace of pores 



the ambulacral groove. 



Description. The evidence of the structure of the ambulacra of the 



irioasteroidea, brought forward in this paper, is based upon ground 



;tions through specimens of Agelacrinus, especially through a well- 

 reserved specimen of Agelacrinus (Lepidodiscus) pileus (Hall) ; as also 

 ipon investigations on a uniquely preserved specimen of Agelacrinus 

 cincinnatiensis (F. Roemer). 



In all cases when the anal pyramid could not be observed, the anal 

 inter-radius was identified from the fact that the rays adjacent to 

 the anus always curve to wards it. Following the nomenclature of 

 Jaekel, the ambulacrum to the left of the anus would be called I, and 

 the remainder counted clockwise II, III, IV, V. 



Viewed externally the ambulacra of Lepidodiscus pileus are seen to 

 be roofed over by an alternating series of plates which are shaped 

 like a bent finger, fig. 9. These do not completely enclose the groove, 

 there being small interspaces between each covering plate. Both 

 longitudinal and transverse section show that these plates also form 

 the sides of the groove. There is no trace of any separate side 

 plates. We may therefore call this upper series of plates " side- 

 covering plates." There are often slight interspaces between them 



