52 CALAMOCEINUS DIOMED.E. 



compact reticular system to the delicate open system of meshwork imme- 

 diately smTouuding the opening of the axial canal (Plate XXI. Fig. 4). 

 The lower face of the upper joint (Plate XXI. Fig. 2) is flat, and the 

 raised ano-ular ridjj-es are less marked, but otherwise it does not differ 

 materially in its structure from that of the upper foce. Tlie quinque- 

 lobular disk (Plate XXI. Fig. 3) has the same reticulation Avith the inner 

 part of the stem joints. It has a delicate open meshwork surrounding the 

 axial canal, Avhich passes gradually into a closer and more compact lime- 

 stone plating towards the periphery and in the wedges forming the angles 

 between the adjoining indistinct lobes. This disk seems to differ in its 

 mode of origin from that of new joints, and I am inclined to look upon 

 this disk as the representative of the anchylosed infrabasals detected in 

 the ApiocrinidfB.* The new joints, if we can judge from the figures I 

 have given, appear at first towards the outer edges of the older rings as 

 delicate limestone spurs extending towards the centre, while in the case 



* With the exception of Eucrimis and Apiocrinus, tlie infrabasals have not been described in the 

 more recent types of Crinoids, but they are known in about half of the pahfozoic Ciinoids. 



The infrabasals of Enorinus, as figured by Beyrioh (Crinoiden d. Muschelkalks, p. 1, Plate I. 

 Fig. 1"), are entirely enclosed and hidden by the upper stem joint, and occupy exactly the same posi- 

 tion as the lobed plate I have described at the summit of the uppermost joint of Calamocriiius. 

 ])e Loriol (Alonog. des Crinoides de la Suisse, p. 7, 1878-79) considers the supplementary basals 

 of Eiicrinus as forming no part of the calyx, but as belonging to the uppermost stem joint, which 

 he has called the " article basal." De Loriol figures iu the Paleontol. fran^aise (Tom. XI. 

 le Partie, Crinoides, 1S8"2-S4) five small plates radially placed upon the centre of the summit of the 

 top stem joint of two species of Millericrinus, M. polydactylus (Plate 110, Figs. 1'^, 2", p. 553) and 

 M. D'Orbignyi (Plate 116, Figs, l^, 1^, p. 500). Tliey do not appear at all upon the exterior of the 

 calyx, but rest upon the central part of the enlarged uppermost stem joint, and are concealed by 

 the basals outside of them. 



Judging from the little we know of the young stages of the dicyclic palreozoic forms as described 

 by Wachsmuth and Springer (Transition Forms in Crinoids, Proc. Phil. Acad., 1878, p. 229), the 

 infrabasals are very highly developed, and gradually become less prominent as the young grow older. 



This seems to show that the infrabasals may have become rudimentary, or have been lost alto- 

 gether in those forms in which it has not been observed. I'he discovery of infrabasals by Bury in 

 the embryo of Comatula (Morphol. Studies, p. 148), and their subsequent resorption, goes far towards 

 making this a plausible explanation of the existence of infrabasals in the older types and their ab.sence 

 in the more recent ones ; although, as Neumayer justly remarks, we should be very cautious in taking 

 the development of Comatula, a geologically recent and quite aberrant type,^as characteristic of that 

 of the Crinoids as a whole. 



The infrabasals of Encrinus, which support the basals, are in Stemniatocrinus represented by a 

 fiat plate. Carpenter is inclined to look upon this as the top stem joint, the arlicle based of Apiocrinus 

 and Millericrinus. This flat plate may have arisen from the anchylosis of the infrabasals, or it may 

 in some cases correspond to the upper stem joint. This is the case in Rhipidocrinus, where the infra- 

 basals are apparently anchylo.sed into a central plate. 



The formation of the centrodorsal in Comatula and in M. Pratti, in which the infrabasals are 

 anchylosed with one or more stem joints, may occur in other cases. 



But as we know the development of Crinoids only from a single type, the representative of a gi'oup 

 with a modified calyx, we should be careful, as Carpenter has already suggested, in making compari- 

 sons based upon iMnbryological dalu with tlie endless and complicated types of the palaeozoic genera 



