GENERA OF THE SUBORDER PENTAMEROIDEA 



183 



Of the species heretofore referred to Conchidium. 

 several must be removed to other genera. C. ktuippi, 

 for example, does not have the elongated and ex- 

 tremely incurved beak of this genus; it is essentially a 

 rather strongly costatc Pcntamrrus and belongs to our 

 Rhip'uiium. Likewise C. Icgoensis Foerste and C. cras- 

 iipititi Hall and Clarke do not belong in Con- 

 chidium, the non-bifurcation of the ribs being of little 

 significance. Savage" has correctly removed C. de- 

 cussatum to the genus Virgiana. 



Genus BROOKSINA Kirk 1922 

 PI. 28, fig. 24 

 Kirk, Proc. U. S. Nat. Mus., vol. 60, 1922, pp. 2-5. 



Genoholotype. — B. alaskensis Kirk 1922. 



Description. Exterior. — Transversely subellipti- 

 cal in outline; hinge-line narrow; cardinal extremi- 

 ties obsolete. Lateral profile unequally biconvex to 

 convexo-concave, the dorsal valve strongly convex, 

 the ventral gently so, flat or strongly concave. Ventral 

 interarea narrow, almost obsolete. Ventral cardinal 

 slopes flat and defined by prominent ridges extending 

 from the beak to the point of juncture of the costellate 

 portion of both valves. Delthyrium probably always 

 open. Dorsal interarea obsolete ; umbo swollen. Sur- 

 face of the valves multicostellate ; shell substance 

 fibrous, impunctate. 



Ventral interior. — Spondylium duplex supported by 

 a long duplex septum. Spondylium narrow but 

 exceedingly deep. 



Dorsal interior. — Septa and lamellae as in Con- 

 chidium and other genera of the Pentamerina:. 



Geologic range. — Upper Silurian of Alaska. 

 Possibly Lower Devonian of the Urals. 



American Species 

 Brooksina alaskensis Kirk 1922 



Asiatic Species 

 Pentamerus of talus Tschernyschcw 1885? (wow Barrande) 



Distinguishing characters. — Brooksina may be 

 differentiated from all other pentamerids by its con- 

 vexo-concave profile and costellate exterior. 



Discussion. — As would be expected, the interior 

 of this genus differs somewhat from other members of 

 the family because of the reversed convexity of its 

 valve. The ventral septum is long and extends to the 

 anterior margin of the spondylium. Toward the front 



"Jour. Geol., vol. 26, 1918, p. 335. 



it is strongly corrugated. The spondylium is exceed- 

 ingly narrow and deep and the anterior margin is 

 rather straight. Postero-latcrally its sides flare outward 

 noticeably where they meet the delthyrial margins. 



In the dorsal valve all parts of the cardinalia can be 

 distinguished as in Conchidiutn and there is a long 

 brachial process which terminates near the anterior 

 end of the spondylium. In a specimen 30 mm. long 

 this process is 9 mm. in length. 



The young of Brooksina are almost equally biconvex 

 and this gives a clue to its ancestry. It certainly must 

 have come out of some Conchidium stock, perhaps out 

 of the " biloculare" line or some form resembling C. 

 nysius (Hall and Whitfield). It certainly did not 

 come out of the "knighti" stock, with which it is 

 contemporaneous. 



Brooksina is in the same stage of development as 

 Cafelliniella and is thus to Conchidium what Cafel- 

 liniella is to Pentamerus. These two forms and 

 Anastrophia are the only representatives of this stage 

 of evolution so far observed in the pentamerids, but 

 other "reversed" forms as aberrant developments are 

 to be expected. 



Genus CYMBIDIUM Kirk 1926 

 T. fig. 34 



Kirk, Proc. U. S. Nat. Mus., vol. 69, art. 23, 1926, p. 2, 

 pi. 1, figs. 5-13; t. fig. 34. 



Genoholotype. — C. acutum Kirk 1926. 



Description. Exterior. — Subelliptical in outline; 

 lateral profile subequally convex, the dorsal valve with 

 the greater convexity. Interareas obsolete; delthyrium 

 open so far as known. Each side of the delthyrium 

 bounded by a broad flattened area. Dorsal beak 

 strongly arched, umbo swollen. Surface multicostate, 

 shell substance fibrous, impunctate. 



Ventral interior. — Delthyrial cavity deep; bounded 

 by a free spondylium which extends for a long distance 

 forward. 



Dorsal interior. — Septal plates widely spaced, rather 

 short. Inner plates exceedingly short. A low short 

 median ridge exists in the middle of the notothyrial 

 cavity. 



Discussion. — Cymbidium is a most unusual brach- 

 iopod and its careful description by Kirk leaves little to 

 be added. Of special interest is the free spondylium, 

 which is a very rare structure in this class of organisms. 

 Careful sectioning by Kirk failed to reveal any trace 

 of a septum or any evidence that the spondylium had 

 previously been sessile. Among the pentamerids Hol- 

 orhynchus is the only other genus having a free 

 spondylium. 



In the dorsal valve the septa are clearly pentameroid 

 although not absolutely typical. Division into three 



