38 



GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA 



(3) Clitambonites ty-pe (see pi. A, fig. 7^ pi. 8, fig. 6; t. fig. 10). — Here the brachiophores 

 are simple blades or flattened rods, but are supported in a unique manner. Shell matter of the noto- 

 thyrial platform is built up conspicuously under the cardinal process and is extended laterally for a 

 considerable distance, forming, with the prominent median ridge and chilidium, an anchor-shaped 

 structure. However, the original or youthful brachiophore is in some specimens clearly distinguish- 

 able from the adventitious shell matter. This type of brachial apparatus occurs in Clitambonites, 

 Vellamo, Gonambonites, Estlandia, Hemipronites, Deltatreta, and Pomatotrema; also in certain shells 

 referred to the Plectambonitidse, as Plectambonites s. s., Plectella, and Ingria. 



Another modification of the clitambonitid type is visible in Productorthis, in which the brachio- 

 phores are at first of the primitive flat blade type, but in later growth adventitious shell is laid down 

 inside and outside of them and likewise around the distal end of the blade, forming cuplike structures. 

 This type, in a somewhat modified form, also occurs in certain dalmanellids, totally obscuring the 

 original brachial apparatus (see pi. 4, fig. 1 5). 



(4) Plectorthis-Hebertella type (see pi. 11, figs. 4, 26). — Here the brachiophores are sup- 

 ported by concave plates which unite with the floor of the valve on each side of or beneath the 



Fig. 10. — Estlandia marginata (Pahlen). Diagram showing brachiophores in place supported by a 



lateral swelling of adventitious substance. 



cardinal process. The socket is defined, not by the outer face of the brachiophore, but by a small 

 concave plate called the fulcral plate^^ located between the outer face of the brachiophore and the 

 wall of the valve. This type of brachial apparatus characterizes the Plectorthidas and Finkeln- 

 burgia (see pi. 13, fig. 16). It is foreshadowed hy Orusia {•wKich.isve.vy close, to Finkelnburgia) in 

 which the brachiophores are supported by subparallel plates. In young Plectorthis the brachiophore 

 plates are commonly discrete but in older forms they unite beneath the cardinal process or with 

 a median septum. 



(5) Skenidioides type. — This is a modification of the Plectorthis brachial apparatus, and proba- 

 bly is derived out of that of Finkelnburgia. The brachiophores are very long and the supports are 

 concave, uniting with a high median septum to form a cruralium. The sockets are defined by con- 

 cave fulcral plates.^" This type is not confined to the impunctate orthids but occurs also in Linopor- 

 ella (see pi. 10, figs. 8, 10, 12). 



(6) Schizophoria type (see pi. 23, figs. 7, 8, 21 ; pi. 24, fig. 3). — Here the brachial apparatus 

 simulates that of Hebertella, but differs chiefly in having vertical or widely flaring and never con- 

 vergent brachiophore supports. The brachiophores are rather long and shaped like the tusk of a boar. 

 The sockets are defined by fulcral plates (see pi. 23, figs. 18, 21 ). This type is seen to perfection in 

 Pionodema, Schizophoria, and Enteletes, and characterizes the Schizophoriidas. 



In the (7) Dalmanella type (see pi. 17, figs. 2, 31 j pi. 18, figs. 23, 29), the brachiophores are 

 flattish divergent blades placed obliquely to the vertical. The socket is formed by the outer sloping 



^1 Cooper, Jour. Pal., vol. 4, 1930, p. 371. 



^^ See Ko7.1owski, Pal. Polonica, pt. I, 1929, p. 48, t. fig. 5, for illustration of this type. 



^^ Cooper, op. cit., p. 374. 



