154 TRANS. ST. LOUIS ACAD. SCIENCE. 



forate the same in order to give an outlet to the ovulum. All these 

 are characteristics which are more developed in one than in an- 

 other, forming so a series of connections which, however, can only 

 be specific and not generic. 



Again, we hardly can find another fossil so subject to abnormal 

 developments as the one in question ; for I have a number of such 

 specimens in my collection, as, for instance, one with six ambu- 

 lacral fields ; one with five ambulacral fields but only four fork 

 pieces ; another with five fork pieces but only four ambulacral 

 fields ; another in which a longitudinal piece is inserted between 

 two fork pieces (see Plate B, Fig. ii) ; one with a half-developed 

 ambulacral field ; another with a spinal excrescence on one of 

 the pelvis plates, and others ; — though all these bear the character 

 of abnormity and never will be mistaken for separate species. 



Pentremites sriNOsus, n. s. (Plate B, Fig. i.) 



Body medium size, subglobose. Pelvis small, representing a 

 moderately convex disc of a little less than J the transverse diam- 

 eter of the body. Foi-k pieces broad and nearly as long as f of 

 the entire length of the body : base portion of the same, sloping 

 abruptly towards the margin of the pelvis ; upper portion of 

 the same, towards the separating suture of the adjoining piece, 

 having on each side, at the apex of the ambulacral fields, a very 

 projecting spine yV of an inch in length. Deltoid pieces small, 

 lanceolate ; its upper acute angle, which projects above the sum- 

 mit, is slightly bent outwards, whereas its lower obtuse angle is 

 bent inwards, forming here a little pit at the junction of the del- 

 toid and fork pieces, Ambulacral fields broad, concave, and very 

 depressed. Lancet pieces a little more than half the width of 

 the field. Poral pieces thin, numbering 9 to yV of an inch. All 

 apertures on the summit rather small and closely arranged. Sur- 

 face ornamented with fine striae. 



Dimensions — Vertical height jV of an inch, width nearly j\ of 

 an inch, width of ambulacral field 2% of an inch. 



This species resembles in its general aspect the Pentremites 

 sulcatus, hut is readily distinguished from the same by the spines 

 on each side of the ambulacral field. 



