94 DAVENPORT ACAnKMV OF NATURAI, SCIENCES. 



stitutes the ujipermost part of the entire l)ody. The sides toward the 

 sinus are elevated and formed into shar[) edges which stand out at riglit 

 angles above the ambulacra. Radial lijjs i)rominent. 



The interradials are not observed in a side view; they are extremely 

 small, and are placed against the tips of two adjoining limbs, within 

 the truncation of the i)eristome, where they form a narrow rim around 

 the spiracles. 



Ambulacra broadly linear, the lateral sides dejiressed, and forming a 

 deep sulcus; the inner portions almost on a level with the forked 

 plates, except near the mouth, where the whole ambulacrum lies deeper 

 than the surrounding plates. 



Spiracles ovate, drawn out in a sharp angle, pointing to and situate 

 close to the mouth. They are placed within the interradial plates, taking 

 up almost their whole surface, leaving exposed a very narrow rim ; four 

 of them are equal, the posterior one is larger and confluent with the anal 

 aperture. Hydrospires and summit ])lates unknown. 



The ornamentation consists in fine concentric lines following the 

 general contour of the plates. 



Of the two specimens from which this species is described, one is 

 nearly perfect and larger than the other. On one side of the smaller 

 one is a crushed mass of slender arms, composed of ]Meces of about 

 e(jual width and height, which possibly may be some of the pinnules. 



Petitremitidea amcricana resembles very closely Peiitremites clavatus 

 Schultze {Penireiuitidea clavata Ether, and Carj).), so closely, indeed, 

 that it might almost be taken for it ; the latter, however, has compara- 

 tively longer basals, is more pentalobate, has wider limbs, with decid- 

 edly convex sides in i)lace of almost straight ones, and the ambulacra 

 extend beyond the general surface. 



Geolof^kal position^ etc.: In the upper portion of the Hamilton 

 group in the Thuntler Bay region of Northern Michigan. The types 

 are in my own collection. 



