252 



FEW GENERA AND SPECIES OF CYSTIDEA. 



[1854. 



diameter. Tlio surfaces of tlio large plates appear to havo been 

 nearly smooth, or only obscurely striated ; but as the specimens 

 are much worn, this appearance may be deceptive. These are 

 the only specific characters that can be given at present. 



Pleurocystites filitexlus. 



Fig. 13. _ Fig. 14. 



Fis 



13. Ventral aspect of an imperfect specimen. 



14. The left rhomb alone. 



The rhombs of this beautiful species are lozenge-shaped, -with 

 straight sides and sharp angles. They are much larger than 

 those of Plcurocystites squamosus, and situated perpendicularly 

 — that is, the longest diagonal of the rhomb extends up and 

 down, and the shortest lies across the fossil. The reverse of this 

 is the order in which they are disposed in the other two species. 

 In all the species, it should be here observed, the left rhomb 

 above is the largest. Fig. 14 is the left rhomb of a specimen 

 of the size figured in 13. All the large plates on the dorsal 

 side have strong rounded ridges radiating from the centres to 

 the corners, and smaller ones between them, which cross the 

 lines of division between the plates at right angles from centre 

 to centre. These are again crossed by lines of growth parallel 

 with the edges, producing a beautiful woven effect. Hence the 

 specific name. The integument consists of about forty angular 

 plates of various sizes. These characters separate this species 

 from the other in a very marked manner. In one locality there 

 are great numbers of the plates and disjointed columns of this 

 species, and it was there that the specimen Fig. 13 was found. 

 It is separated from the matrix, but although the back exhibits 

 one rhomb and the character of the striation, yet it is otherwise 

 so much distorted that a figure of it would convey but little 

 instruction. I have not been able to ascertain clearly the size o 

 the other two rhombs. 



Pleurocystites robustus. 

 Fig. 15. 



Of this species I have only the fragment liero figured, but it 

 is so widely different from the others, that there can be little 

 doubt of its being distinct. The rhombs are nearly in the shape 

 of a spherical triangle, one side crossing the suture above and 

 one of the angles being upon it below. Here, too, the left one 



is the largest. They are excavated into a deep hollow with a 

 rounded bottom, the longer axis lying across the fossil. They 

 are also surrounded with an elevated border. The plates near 

 the edges are marked with fine stria? at right angles to the sides, 

 but the ridges proceeding to the corners are barely perceptible. 

 There are also several faintly-perceived crenulations parallel to 

 the edges. An elongated tumour lies between the right rhomb 

 and the vertical suture in the centre. Altogether, this is a more 

 vigorous species than either of the two former. 



Besides these, there are several specimens exhibiting only the 

 back, but very perfect, which will constitute either one or two 

 other species. Until the other side can be seen, they can hardly 

 be classified as one, and yet they are clearly distinct from the 

 above. It is only within the last four months that specimens 

 were discovered showing the structure of the ventral side of this 

 genus, and since then there has not been time to study them 

 minutely. It may be that an aperture will yet be found situated 

 somewhere in the integument near the centre, but at present 

 appearances are against it. 



These are all the Cystida? in my possession with poriferous 

 areas clearly developed, and which have been found in a suffi- 

 cient state of perfection to admit of their being described; but 

 there are detached plates frequently found here in the upper 

 layers of the Trenton limestone, which exhibit the remains of 

 these organs of a form different from any of the above. They 

 prove nothing, however, more than the fact that other species of 

 Cystidea are imbedded in the formation. There are seven other 

 species, of which I have some very good specimens, which differ 

 widely from Glyptocystites and Pleurocystites, and of these I 

 shall probably prepare another paper before the end of the ses- 

 sion. They are without poriferous areas, and some of them 

 approach the Spheronites in form, but are furnished with fimbri- 

 ated arms. 



It may be proper here also to notice a remarkable Crinoid, 

 which conspicuously displays upon its surface upwards of twenty 

 small sub-triangular spaces perforated with elongated pores, re- 

 sembling those of the rhombs of the Cystida?. The cup is small 

 and conical, composed of three rows of plates. The first series, 

 or pelvis, consists of five pentagonal pieces, the second series of 

 five, four of them hexagonal and one heptagonal, alternating 

 with those below. The scapula? are also five, and of a heptagonal 

 shape. They alternate with the second series. On one side, 

 between two of the scapula?, there are either one, two, or three 

 small plates ; but owing to the circumstance that all the speci- 

 mens (four in number) happen to be badly preserved or muti- 

 lated at this point, they cannot be ascertained. 



These poriferous areas are each formed of the three angles of 

 three-contiguous plates. From the point of each plate a pore 

 extends towards the centre, and all the others on that plate are 

 parallel to this central pore. In each area, therefore, they run 

 in three directions, and are not at right angles to the sutures 

 between the plates, as in the Cystidea. Figs. 16 and 17 will 

 explain this arrangement with greater clearness than a written 

 description. 



Fig. 16. 



