BRACHIOPODA. 377 



Arthropomata.] 



than one-half the size attained by this species as found at Middleville, New York, 

 and since they are constantly smaller, the varietal name minor is here applied to 

 them. The growth lines are conspicuous in some specimens, while in others they 

 are nearly obsolete. The muscular markings are undefined, owing to the tenuity of 

 the valves. 



Formation and locality. Not uncommon near the base of the Galena shales at St. Paul; associated 

 with Clitambonites, three miles south of Cannon Falls and at Kenyon, Minnesota. Also in the upper part 

 of the Trenton shales at Chatfleld. 



Collectors. A.. D. Meeds and C. Schuchert. 

 Mus. Beg. No. 7279. 



Subclass ARTHROPOMATA, Owen. 



Order PROTREMATA, Beecher. 



Family CLITAMBONITID^, n. fam* 



Genus CLITAMBONITES, Pander. 



1830. Klitambonites, PANDEK. Beitrage zur Geognosie des russischen Reiches, p. 70. 

 1892. Clitambonites, HALL. Palaeontology of New York, vol. viii, pt. i, p. 233. 

 Orthisina, d'ORBiGNY (1847) and subsequent authors. 



Description: "Shells with a subsemicircular marginal outline; convex or sub- 

 pyramidal in the typical group. Hinge-line straight and forming the greatest 

 diameter of the shell. Pedicle [ventral] valve elevated, cardinal area high, vertical, 

 or sometimes incurved and crossed by a broad delthyrium, which is normally covered 

 by a convex, perforate deltidium. On the interior of the valve the dental lamellae 

 are very strongly developed, converging and uniting in the median line before 

 reaching the bottom of the valve, thus forming a spondylium, which with the del- 

 tidium encloses a conical subrostral vault. This plate is supported by a median 

 septum extending for about one-half the length of the valve. [Adductor, diductor 

 and adjuster scars occupy the upper surface of the spondylium.] In the brachial 

 [dorsal] valve the cardinal area is considerably developed and the delthyrium filled 

 by a conspicuous callosity, against the inner side of which the simple orthoid cardinal 

 process abuts. The dental sockets are large, the crural plates low and continuous 

 with the edges of the delthyrial callosity. A thickened transverse area is formed 

 in the umbonal region by the union of the inner portions of the crural plates with 

 the cardinal process, and thence a broad median ridge is continued forward through 

 the muscular area, which is sharply defined and quadripartite. External surface 

 covered with radiating striae. Shell substance impunctate. 



This family will contain PratnrtMs. Hall, ClitamTjonites, Pander, Hcmiprcmites. Pander, and Scenidiitm, Hall. Waagen 

 (Pal. Indica, vol i, p. 576, 1884,) proposed the sub-family OrUiMnas for the genus Orthisina. D'Orblgny's family OrtMsidae 

 contains Strophomena, Orfhte, and Orthisina Clltambonites, genera now referred to three distinct families. 



