no 



GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA 



lamellx strong, adductor field lanceolate, enclosed by 

 the subcrescentic diductor scars. Pallial trunks as in 

 Orthis s. s.y bounding subreniform ovarian impressions. 

 In the dorsal valve the brachiophores are simple rods 

 supported by lateral growths of shell substance as in 

 Vellamo. Cardinal process simple. Adductor field 

 subflabellate, anterior scars the smaller. Inside the 

 margin of the valve is a thick subperipheral rim. 



To this genus also belong Orthisina grandwva Bill- 

 ings, and Clkatnhonites semiconvexus Poulsen. 



Horizon and locality. — Ordovician (Cana- 

 dian), SW 14 Sec. 2, T 1 S, R 1 W, about 4 miles 

 east of Hennepin, Oklahoma. 



Genus POLYTCECHIA Hall and Clarke 1892 



Hall and Clarke, Pal. N. Y., vol. 8, pt. 1, 1892, p. 239, pi. 



7a, figs. 26-30, t. figs. II, 12. 

 Horn. Waagenia Hall 1889. 



Genoholotype. — Hemifronkes aficalis Whitfield 

 1886, Bull. Amer. Mus. Nat. Hist., vol. 1, p. 300, 

 pi. 24, figs. 1-5 (non Orthis? apicalis Billings). 



Original description. — Shell small, subtrihedral in 

 contour. Hinge-line straight, about equaling the diameter 

 of the shell. Pedicle-valve with a high, nearly vertical car- 

 dinal area marked with oblique striations parallel to the 

 lateral margins. Delthyrium covered by a narrow, convex 

 plate; the presence of a foramen not determined. On the 

 interior the dental lamellx are widely separated, and 

 descend along the umbonal cavity for a short distance verti- 

 cally, thence bending sharply inward and meeting at a low 

 angle in the median line; thus forming, with the del- 

 tidium, a conspicuous subrostral vault. This inner spoon- 

 shaped plate, spondylium, is supported by a stout median 

 septum, and two smaller lateral septa, which meet it at the 

 lines of angulation ; the former of these extends for the 

 entire length of the plate, while the latter is free from the 

 accessory septa near its anterior edge. The umbonal cavity 

 of the valve is thus divided into five chambers, and in the 

 lateral chambers there is still another septum, lower than 

 the rest and not extending to the spondylium. The 

 brachial valve is shallow and depressed-convex, with a nar- 

 row cardinal area. The delthyrium is very broad with a 

 partially developed covering, the dental sockets are widely 

 separated, the crural plates narrow and nearly parallel to 

 the hinge-line. The cardinal process is simple, linear and 

 quite prominent, and at its union with the crural plates is 

 a subtriangular thickening which is supported by a low 

 median septum. Surface covered with fine, elevated, radiat- 

 ing strise, without evidence of median fold and sinus. 



Geologic RANGE. — Canadian (Beekmantown) and 

 Chazy? 



American Species 



Hemifronites aficalis Whitfield 1886 

 Polytoechia symmetrica Butts 1926 

 P. ? oaiensi's Butts 1926 



Discussion. — The multicamerate apical ventral in- 

 terior of this genus is characteristic of it, and shows 

 a convergence toward similar structures in Gonambo- 

 nkes s. s. and Clarkella. The genus, however, can 

 not be placed in the Syntrophiidae because of its ex- 

 ternal sculpture, shape of the valves, and internal 

 features of the dorsal valve, which are evidently close 

 to those of Vellamo or Deltatreta. The presence of a 

 deltidium and chilidium are further features linking 

 this genus intimately with the Clitambonitids or 

 Deltatretidas. 



Polytcechia appears to be an American genus only, 

 and for this reason its relationships to Clkanibonkes s. s. 

 are doubtful. The ensemble of the shell ornamenta- 

 tion, profile, etc., suggest Deltatreta, and were it not 

 for the multicamerate apex it would definitely belong 

 here. The position we assign the genus is tentative. 



Family CLITAMBONITID^ Winchell and 

 Schuchert 1893 



Divergent specialized Protremata derived out of the 

 Orthacea, having well developed deltidia and chilidia. 

 In the ventral valve the muscles are usually borne on 

 a spondylium simplex. In the dorsal valve the car- 

 dinalia are of the Orthis type, supported by lateral ad- 

 ventitious shell growths. Ventral, pallial, and ovarian 

 markings essentially the same as those of Orthis. 



Discussion. — The members of the Clitambonitidas, 

 prolific in variable individuals, have until recently been 

 classified in the Pentameracea, but Schuchert in 1929^' 

 made them the basis of a new superfamily, the Clitam- 

 bonacea. This he did when he saw that the stock 

 had a genetic origin independent of the Pentameracea, 

 as is plainly shown by their different type of spondylia. 

 It is true that the Clitambonacea are closely related to 

 the Orthidje, but as they are not in the main line of 

 brachiopod evolution, being a specializing stock, and 

 since most of them have peculiar deltidia and as a rule 

 strong chilidia, and more or less well developed simple 

 spondylia, it is thought advisable to recognize these 

 tendencies and to separate them from the Orthacea as 

 another superfamily, Clitambonacea. 



Ventral valve, — The chief points of departure from 

 the usual orthid structure in the Clitambonitidas are 

 in the spondylium and deltidium. The former is a 

 spoon-shaped plate usually resting on the floor of the 

 valve at its posterior end but elevated on a median 

 septum at the front end. In a few forms the septum 

 may be absent or may be so abbreviated as to be virtu- 

 ally absent. The junction of the upper ends of the 

 spondylium with the under side of the palintrope is 

 marked by a distinct suture at which the growth-lines 

 of the inner surface of the spondylium end abruptly. 

 The delthyrial margin is an elongate triangular area 



"Foss. Cat., Pars 42, p. 15. 



