NATURAL SCIENCES OF PHILADELPHIA. 77 



Surface very finely granular. [Impressions at the corners of the plates 

 also sometimes cause the appearance of a tendency to form a short obtuse 

 radiating rib at each side of some of the plates, but this character is too faintly 

 marked to attract attention. 



Height of body about 1-70 inch, breadth, 1-60 inch. 



The most marked feature of this species is its rather obovate form, produced 

 by the narrowness of its lower part and its convex sides, and sliglit contraction 

 near the top. The narrowness of its lower part results from the small size 

 of its basal, subradial, and first radial pieces. The deeply sunken character 

 of its base also contributes to the same result, as it is not near so wide as it 

 would be if its pieces extended out horizontally from the column, instead of 

 being folded down with their backs against it. It seems to resemble G. tub- 

 erculostis, Hall (sp.), more nearly in general appearance than any other species 

 known to us, but will be readily distinguished, not only by its different form, 

 but by the proportionally smaller size of its basal, subradial, and first radial 

 pieces, but more particularly by the structure of its false arms, which are each 

 composed at the base of six ranges of pieces, instead of only two. 



Locality and Position. Upper division Burlington Group, Burlington, Iowa. 

 Lower Carboniferous. No. 379 of Mr. Wachsmuth's collection. 



Genus LEPIDOCENTRUS, Muller (?) 1856. 



Entire form unknown. luterambulacral plates rather thin, very irregular 

 in size and form, all strongly imbricating apparently from below, and toward 

 the lateral margins of the areas ; arranged in five or more rows, only the outer 

 two of which seem to be continued to the extremities of the areas, all occupied 

 with generally obscure secondary granules, and most of those on the lower (?) 

 side of the body also provided with large primary central tubercles and 

 spines ; primary tubercles sometimes showing'a small pit in the top, surrounded 

 by two smooth rings, separated by an annular furrow, and all without any 

 ring, depression or prominence around the base ; on the upper (?) side of the body, 

 only the marginal rows provided with primary tubercles. Ambulacral areas 

 wide, and occupied by six or more irregular rows of unequal, irregular pieces, 

 some of which are as large as the smaller interambulacral plates, and all strongly 

 imbricating in the opposite direction from the interambulacral series ; each 

 pierced by two pores, and the larger ones usually marked with one or two 

 additional pits, which, with the pores, are surrounded by a large circular im- 

 pression, while some of them sometimes show a tubercle intermediate in size 

 between the primary and secondary series. Apical disc unknown, but a single 

 rather large plate believed to belong to it is seen to have six or seven pores 

 circling around near its outer margin, and a small tubercle in the middle. 



This type is related to several of the other palteozoic genera, but cannot 

 be properly referred to any of them. In the great irregularity of its interam- 

 bulacral plates, both in size and form, as well as in the absence of primary 

 spines, excepting on the marginal rows (at least on the upper (?)side of the body) 

 it seems to be very similar to Perischodomus of McCoy. It differs, however, 

 clearly from that group in the much greater breadth of its ambulacral areas, 

 the greater number of ranges of ambulacral pieces and their greater irregular- 

 ity, as well as in the much larger sizes of some of them, and their peculiar 

 circular impression around the two pores. If Perischodomus has its plates 

 not imbricating, that would also be another important difference, but although 

 that character is not mentioned in the description, we suspect it may really 

 exist, because we find it to occur in all the analogous types in this country. 



From Lepidechinus it is also readily distinguished, by the much greater breadth 

 of its ambulacral areas, and its more numerous rows of ambulacral pieces and 

 pores, as well as by the larger size and the other pecularities of these species. 



In the breadth of its ambulacral areas, and the number of rows of pieces 

 occuping the same, it is more nearly related to our Lepidesthes ; but it differs 



1869.] 



