new Species of Crinoids. 167 



but portions of it are in situ, and there are also numerous 

 detached plates in the interior of an individual whose roof has 

 collapsed. It was composed of a solid massive layer of 

 numerous small polygonal but relatively thick plates, very 

 intimately united together, so that the partitions between 

 them cannot in many instances be detected *. Their contact- 

 surfaces in some cases show minute longitudinal ridges and 

 grooves of a similar character to those of the inter brachial 

 plates, but finer ; in other cases the contact-surfaces appear 

 to be quite smooth. The plates throughout the vault are 

 about "9 millim. in thickness, and thus nearly double as thick 

 as the interbrachial plates. Those of the peripheral area 

 (PI. VI. fig. 10) are not more than 1 millim. in width; 

 their upper surfaces are now relatively smooth, but on each 

 there are from one to four spine-bearing tubercles of two 

 different sizes. The plates of the central portion of the vault 

 are thickly covered with minute blunted fixed spines or 

 tubercles, with some articulated spines as well, and also 

 short sinuous ridges (PI. VI. figs. 11, 12). I am unable to 

 ascertain definitely the form and number of the plates in this 

 central portion ; some were clearly larger than those of the 

 peripheral area, and there is a detached united mass, evidently 

 belonging to the central area, which may be of only two or 

 three large plates ; but no divisions are perceptible in it. 



Tubercles and Spines. — The tubercles are minute truncated 

 cones which project slightly from the surface of the plates. 

 Their narrow summits are perforated by a minute circular 

 aperture or pit, from T to T5 millim. in width ; their sides 

 gradually enlarge towards the base, which varies from -3 to 

 '5 millim. in width, and merges into the general surface of 

 the plate without any distinctive collar or depression. In the 

 best-preserved examples there is no prominent rounded knob 

 at the summit of the tubercle. They appear to be very liable 



* Prof. Williams (op. cit. p. 88) considers it probable that the detached 

 spine-bearing plates described and figured under the name of Lepidocen- 

 tru8 eifeliawus by Job. Midler from the Devonian at Rommersheim, may 

 have belonged to the vault of a crinoid similar to Hystricrinw. From 

 Midler's description of these plates in the Abhandl. de'r konigl. Akad. d. 

 Wiss. Berlin, 1856, p. 259, it appears that their edges are bevelled and 

 adapted to fit over each other like fish-scales. This character, and the 

 fact that nearly similar plates have been discovered in position, forming 

 part of the test of a Paleechinus, by Schultze (l)enkschr. konigl. Akad. u. 

 Wiss. Wien, Bd. xxvi. 1807, p. 124, Taf. xiii. fig. 1), satisfactorily show 

 that Midler's specimens could not have belonged to a crinoid. Their 

 relatively large size also renders it improbable. At the same time it 

 should be acknowledged that the simple form of the tubercles and spines 

 and their attachment to each other in Lepidocentrus and also in Pholido- 

 cidaris, Meek and Worthen, are of the same character as in Hystricrinus. 



