694 GEOLOGICAL SURVEY OF IOWA. 



Pentremites synimetricus ( n. s.). 



PLATE XXV. FIG. 14. 



Compare Pentremites pyriformis, etc. 



BODY narrow ovate, with a pointed and obtusely angular 

 base ; greatest diameter about one-third the distance upwards 

 from, the base, very gradually tapering above, with summit 

 rounded or subtruncate. Basal plates small, forming a small 

 pentapetalous cup, with one or two joints of the column 

 usually attached. Radial plates narrow below, expanding to 

 the base of the pseudambulacral areas and thence continuing 

 with the sides parallel, prominently angular in the middle 

 below the furcation. Interradial plates very long lanceolate, 

 not reaching the summit within the space of four or five 

 poral plates. Pseudambulacral areas long lanceolate, reaching 

 two-thirds the entire length of the body, convex ; the poral 

 plates of each side turning slightly upwards along the median 

 line, and divided along the centre of each side by distinct 

 sutures. About forty plates on each side, in a specimen of 

 medium size. Mouth small, pentagonal : ovarian apertures 

 round-ovate, closely arranged, that on the anal side distinct- 

 ly larger. 



SURFACE finely and beautifully striated parallel to the 

 margins of the plates. 



This species differs from the P. pyriformis in the less prominence of the angles at 

 the base of the pseudambulacral areas, while the median angle of the radial plate is 

 more conspicuous : the proportions of length below and above the greatest diameter 

 are very distinct in the two species. The pseudambulacral areas are more convex; and 

 in a specimen of P. pyriformis of the same size as the one figured, there are no more 

 than thirty-five poral plates on each side of the area ; and in none of the specimens 

 of that species examined, have I been able to distinguish the longitudinal suture 

 which divides each side of the area into two equal parts. 



The specimen fig. 14 was originally given to illustrate a variety of form in the P. 

 pyriformis ; but the constancy of the characters presented in a large number of 

 specimens seems to sanction its sop iration as a distinct species. 



Fig. 14. View of a specimen of medium size. 

 Geological formation and locality. In the Kaskaskia limestone, Kentucky. 



