ART. 19. A NEW PENTAMEROID GENUS — KIRK, O 



'J schernyschew.* Thi.s form Tschernyschew refers to Pentameinis op- 

 tatus Barrande, a species to which it certainly does not seem to 

 belong. So far as the figures go, it is quite possible that the species 

 is referable to Broohsina. This s]iecies is found in the Lower De- 

 vonian of the Urals. 



BROOKSINA ALASKENSIS, new species. 



This species is represented in the collections by two or three hundred 

 specimens in a fair state of preservation. The specimens were found 

 in residual clay resulting from the surface decomposition of impure 

 limestone along a narrow, slightly shattered fault zone. As a rule 

 the shell substance of the valves is poorly preserved. The leaching 

 of the water either removed the calcareous test or loosened it to 

 such an extent that it exfoliated badly. Patches of the test in good 

 preservation are usually found on each specimen, but individuals 

 which preserve the test in its entirety are relatively rare. The speci- 

 mens are not distorted, however, and form excellent material for 

 study. The internal structure may easily be examined, either in cross 

 section or by splitting the specimens longitudinally. The material 

 aA^ailable for study furnishes an unusual series of growth stages. 

 The smallest individual found measures but 3.0 mm. in length by 

 3.5 mm. in breadth. The largest specimen measures 40.0 nmi. in 

 length by 45.0 mm. in breadth. Another large specimen of the same 

 approximate width has a length of 36.0 mm. 



Brooksina alaskensis in the case of the largest individuals lias a 

 suboval outline when view^ed either from the dorsal or ventral side. 

 In smaller specimens, and these may be taken as the average, the 

 anterior margin has the outline of an oval segment, but the posterior 

 portion has a subtriangular outline. In side view the ventral valve 

 ranges from decidedly concave to slightly convex in the portion an- 

 terior to the beak, the latter condition obtaining in specimens above 

 the average in size. The dorsal valve is highly convex, the outline 

 being a smooth even curve except in the apical portion, Avhich is 

 abruptly incurved. The preponderance of the dorsal valve over the 

 ventral, both in size and convexity, is marked. 



The beak of the ventral valve culminates in a sharp point and is 

 sharply refiexed. From the beak and extending to the lateral margins 

 of the valve are lines diverging at somewhat variable angles, but 

 averaging about 120°, that mark a sharply defined offset or shoulder 

 in the valve. Dorsad to these lines is a concave or flattened pseudo- 

 cardinal area. There is usually a well-defined median sinus extending 

 from the beak to the anterior margin of the ventral valve. Lacking 



* Tschernyschew, Th., Die P'auna des Untern Devon am West-Abhansre des Urals, M4m. 

 Comit* Gfeologique, vol. 3, No. 1, pi. 7, fiff. 94, a, 6. o. 



