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stronger and diverge from the suture ; strise on the inter- 

 radial plates diverging from the centre. 



Length, from one-fourth to three-fourths of an inch. 



Locality. — Spergen Hill ; Bloomington, la. 



In young specimens the base is more extended and the 

 poral pieces much fewer than in older specimens. 



Associated with this species, and having a similar general 

 aspect, I have observed a single specimen, having a length 

 of three-fourths of an inch, of an obtusely quadrangular 

 form, and having but four pseudo-ambulacral areas, one of 

 them being much wider than the others. There are, how- 

 ever, five ovarian openings at the summit. This appears to 

 be an individual where the two adjacent sides of the radial 

 plates has never been developed, while at the same time an 

 effort has been made to preserve the symmetry of the ova- 

 rian openings. 



TEREBRATULA, Lhwyd. 



Terebratula turgida. — Shell longitudinally ovate, often 

 extremely gibbous,emarginate in front,dorsal valve most con- 

 vex in the middle, having a sinus extending to the base of 

 the shell ; beak large, rounded and prominent, incurved and 

 pointed, with an oval or subcircular foramen just above or 

 in the extremity. Yentral valve most convex in the mid- 

 dle or near the front, with or without a short sinus, in which 

 is sometimes a short and obscure fold. Surface marked by 

 strong concentric lines of growth ; and near the front, in 

 some shells, are strong wrinkles or folds which distort the 

 form of the shell. 



Length, *16 to '32, width *13 to '27 of an inch. 



Locality. — Alton, 111. ; Bloomington ; Spergen Hill, la. 



1 Terebratula Formosa. — Shell longitudinally oval-ovate ; 

 dorsal valve more convex in the middle and upper part ; beak 

 extended upwards, prominent, incurved ; valves compressed 

 near the front, which is neatly rounded, the margin present- 

 ing a slight undulation ; sometimes sinuate in front. Sur- 



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