71 G DE. W. B. CAEPENTER OJN' THE STEUCTUEE, PHYSIOLOGY, AND 



(Plate XXXVI. fig. 3, c, d), has a well-marked triangular form, presenting three articulav 

 faces ; of these the central or internal looks towards the external face of the Second radial, 

 v.hilst the face that looks obliquely outwards on either side serves as the base of an Arm. 

 The internal face (fig. 8, a) corresponds very closely to that with which it is articulated, 

 being divided, like it, by a vertical ridge, that also passes round the opening of the radial 

 canal, into two lateral fossa?. When avc look at this articular margin of the Third Eadials 

 from the dorsal side (d), we observe that its two lateral portions slope away in some degree 

 from the median prominence ; and this is also seen when we look at the articulation in 

 section (Plate XXXIV. fig. 1) or on its ventral aspect (fig. 2). Hence, when the opposed 

 ridges of the Second and Third Radials are in contact with each other, the third radial 

 would seem to have some power of lateral movement upon the second. As no muscles, 

 however, pass between the second and third Eadials, which are connected by ligaments 

 only, such movement, if it really exists, can only be attributed to the general contractility 

 of the soft parts by which these plates are invested. From the upper margm of the 

 internal face (a) we see projecting a pair of lamellse {d, d) which do not form part of the 

 surface of articulation with the second Radial, but which enter into the two oblique 

 sui-feces of articulation with the first Brachials. Each of these last faces (b) is formed 

 upon the plan just now described as presenting itself in the opposed articular surfaces 

 of the first and second Eadials, having the transverse ridge (a, a), the fossae for the 

 interarticular ligaments {h, b), the fossa for the elastic ligament (/, /), and the muscular 

 fossae (c, c) with their tliin vertical lamellae, the lamella (d') that rises from the distal 

 angle being common to both the oblique faces. The dorsal surface of this plate (d) 

 presents no marked peculiarity; but on the ventral (c) a considerable inequality is occa- 

 sioned by the projection of the three vertical lamella?. The radial canal, as we should 

 expect, here divaricates, one branch j)assing on to either arm (Plate XXXIV. fig. 1, e). 

 39. No trace of Interradial plates shows itself in the variety of Jntedon rosaceus with 

 which I am most familiar, — that, namely, which occurs in the estuary of the Clyde, in 

 Strangford Lough, and in Kirkwall Bay. But in specimens from Ilfracombe and from 

 Plymouth Sound of what, from their likeness in all other respects, I cannot but regard 

 as belonging to the same specific type, I find certain small plates in the angles between 

 the Second and Third Eadials, which are obviously those referred to by Mr. J. S. Miller 

 in his Comatula Jimbriata from Milford Haven as "intercostal plates or joints'". 

 Although MiLLEK figm-es only one such plate in each angle (which may be readily 

 understood from his lia\ing only employed a low magnifying power in examining it), 

 yet I find that, generally speaking, it is resolvable by the Microscope into a cluster of 

 three or four small plates (Plate XXXIII. fig. 7, b), though it is not unfrequently found 

 to consist of an aggregation of several minute bodies (fig. 7, a) scarcely larger individually 

 than the fragments of calcareous reticulation often occurring in the ventral perisome 

 (Part II.). Moreover I learn from Professor Wt\'ILLE Thomsox that in a specimen recently 

 dredged ofi" Shetland by Mr. Baelee, these plates were present in three of the angles, but 

 ' Natural History of the Crinoidea, Frontispiece, fig. 2, G. 



