1854.] 



NEW GENERA AND SPECIES OF CYSTIDEA. 



251 



ingly small, and more than a thousand in number, -while in an- 

 other they are much larger, and only about forty of them. In 

 the Cystidea, as also in the Crinoidea, the part resting upon the 

 column is considered to be the dorsal pole, and the upper part of 

 the cup the ventral ; but in this singular fossil the ventral region 

 appears to have been displaced from its normal position, and drawn 

 down, as it were, to one side, until it reaches the top of the column. 

 This is not a deformity of a single individual, but as it is seen in 

 all the specimens (clearly constituting several species), it must be 

 regarded as a permanent character of the genus. 



It must have been nearly flat, one of the broadsides being regu- 

 larly formed of large plates, and the other covered by the integu- 

 ment. The former may be called the back, and the latter the 

 ventral side. The following is the arrangement of the plates. 

 See Figs. 9 and 10. 



On the upper joint of the column rest four pelvic plates. Two 

 of these are pentagonal, or hexagonal, and spread away from each 

 other in the form of the capital letter Y, and in the angle thus 

 formed is placed the large central hexagonal plate of the second 

 series. The two oiher pelvic plates are situated one on each 

 side, and partly under the former. They do not unite on the 

 other side and form the cup-shaped pelvis of the ordinary Cys- 

 tidea, but spread out, wing-like, from the sides of the column. 

 Each sends out a slender projection at the bottom, which clasps 

 round or rests upon the upper joint. Outside of these, again, 

 are two other small plates, one upon each wing, making in all 

 sis in the basal series. 



In the second range there are three large plates, one in the 

 centre hexagonal, with a heptagonal plate on each side. 



The third series contains four large plates, elongated vertically. 

 One of these on the right hand of the centre is pentagonal, the 

 other on the left hexagonal. They are narrowed above to cor- 

 respond with the decreasing dimensions of the body, which here 

 begins to contract. The other two plates of this row are either 

 heptagonal or sligtly octagonal, and in their upper extremities 

 they fold round the bod)', and unite on the other side by narrow 

 projections, which arch over the great oval opening, as may be 

 seen in Fig. 10. All those plates at the edge are folded under, 

 so as to form a border round the ventral region on the other 

 side. Above these are ten smaller plates, which close the sum- 

 mit, and form a solid support for the arms. 



These arms are each formed of a series of double joints. "When 

 viewed from the rear, as in Fig. 9, or from the front, as in Fig. 

 10, these joints appear each to be formed of a single piece; but 

 when seen from the outside, as in Fig. 11, they are double. On 

 the inside, a double row of minute projecting tooth-like ossicula 

 follows the suture down each arm, and crosses the summit between 

 them. There is a figure on plate 2, opposite p. 51, in Miller's 

 Natural History of the Crinoidea, which shows six fingers of 

 Pentacrinus caput Jledmce attached to the hand, four on one 

 side, and two on the other, which exactly resemble the arms of 

 this species. There, too, it will be observed, a double row of 

 ossicula run up the inside ; and these appendages upon this Cys- 

 tidean appear rather to be of the nature of tentacula than true 

 Crinoidal arms. 



The only apertures visible in the specimens yet procured are 

 a small one immediately below the summit on the ventral side 

 on the suture between two small plates, and another at the base, 

 on the right side of the pedicle (Fig. 10), where the border round 

 the great oval opening is excavated into a channel for the passage 

 of what appears to have been a proboscis of small plates. Between 



the arms, the rows of ossicles which fringe the inside cross from 

 base to base, and it may be that at this point they form a valvu- 

 lar apparatus above the mouth, as in the last species. If this be 

 so, then the orifice immediately below is without doubt an excre- 

 tory aperture, and the other at the bottom may he regarded as 

 ovarian. 



There are three pectinated rhombs, one at the base, half of 

 which is situated on the right pelvic plate, and the other half on 

 the central hexagonal plate of the second series. The other two 

 are situated, one on the left pair of plates in the third series, the 

 other on the right. They are entirely open — that is, the elon- 

 gated pores cross from one side of the rhomb to the other at 

 right angles to the suture, without being concealed by an imper- 

 forated space in the centre, as in Glyptocystites. When the 

 rhombs of the latter are ground down, as before mentioned, they 

 are then open, as in the present genus. 



As this strange inhabitant of the old Silurian ocean consisted, 

 properly speaking, of but one side, constructed on the normal 

 pattern of the Cystidea, I propose, as a name for the genus, 

 Pleurocystites, from the Greek Kt«ms and uxtvpov. There are 

 clearly several species, but I shall only recognise three at present. 



Pleurocystites squamosum. 



Fig. 10. Fig. 11. Fig. 12. 



m 



COSSS^T^ 



Fig. 9. Dorsal view of a large specimen. 



" 10. Ventral view of a small do. 



" 11. Outside of arm, showing the double joints. 



" 12. Inside of do., showing the rows of ossicula. 



In this species the rhombs are small in proportion to the size 

 of the body, and of an oval shape, the greater axis of the ellipse 

 crossing the sutures between the pairs of plates upon which they 

 are situated at right angles. The integument is composed of a 

 vast number, more than a thousand, minute, scale-Eke plates, 

 mostly hexagonal, and less than the fiftieth part of an inch in 



