A. Olsson — New Genus of Palceechinoidea. 443 



Lepidechinoides ithacensis sp. no v. 



Ambulacral plates about three to each of the adambulacral 

 plates bordering them. Each plate perforated in the middle 

 by a pair of pores set close together. Arrangement of plates 

 alternate and imbricated aborally. Interambulacrum consist- 

 ing of six columns of rather large plates imbricated ad orally. 

 Second column of plates larger than the adambulacral plates. 

 Adambulacral plates rhombic or pentagonal in shape and per- 

 forated near the ambulacral edge. The inner series of inter- 

 ambulacral plates more or less hexagonal in shape, except 

 those on the extreme ventral surface, which are small and 

 scale-like. These plates pierced in the center except those on 

 the ventral surface. On the oral surface a few of the ambu- 

 lacral plates are pierced by a pore each on their adjacent ends, 

 but they are confined to this region. Secondary spines small 

 and striated and dilated at the base for attachment. 



Length 2-5 cm 



Width 4 cm 



Width of ambulacral area 4 mm 



Width of interambulacral area at middle l7 mrn 



After a careful study of the specimen and of the descrip- 

 tions and figures of the other three genera in the family, 

 viz. — Lepidocentrus Miiller, P erischodomus McCoy, and 

 Lepidechinus Hall, the following points of resemblance and 

 differences were made out. In 

 the imbrication of both areas it 

 approaches Lepidechinus and 

 Perischodomus and differs from FlGS - 3 > 3 - 



Lepidocentrus, in which the am- 

 bulacra! plates are inflexible. 

 As in Lepidechinus, the initial 

 plate 1/ is retained. It is small 

 and irregularly foursided, its 

 ventral apex is sharp and does 

 not appear to have suffered from 

 resorption. This specimen in its 



possession of only six columns Figs. 2, 3. Secondary spines 



of interambulacral plates is not enlarged ten diameters. 



so highly accelerated in its de- 

 velopment as the genus Lepide- 

 chinus and the species Lepido- 

 centrus mulleri Schultze. In Lepidechinus rarispinus Hall 

 the introduction of new plates is so rapid that each initial 

 plate touches the next. In Lepidechinoides the plates 2, 3, 4 

 and 5 follow each other rapidly, as shown in figure, but between 

 plates 5 and 6 there are three intervening plates. The exact 



