4 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. GO. 



stage in the ontogeny is there a strong resemblance to Conchidiunn. 

 This lack of resemblance is striking when young stages of Brookshia 

 and Gonchidiv/m are compared. Broohsina may not, I believe, be 

 considered a derivative of Conchidium proper. Associated with 

 Brooks'ina are G onchidium-X^ko, forms that have the general char- 

 acteristics of Brooksina without the reversal of valve proportions and 

 complementary peculiarities. It may be necessary to assign these 

 forms to Gonchidium for want of precise definable distinctions war- 

 ranting their separation from that genus. It is hoped that with more 

 material available and a better general knowledge of the Pentam- 

 eroids it will be possible approximately to establish genetic lines and 

 place these aberrant tj^pes in their proper niche. 



Undoubtedly the genus to which Brooksina bears the closest super- 

 ficial resemblance is Gapellmia. This genus was described in 1894 by 

 Hall and Clarke (1894, p. 248) and is represented by one species 

 Gapellinia tnira Hall and Clarke from the Silurian dolomites of 

 Wisconsin. As pointed out by Hall and Clarke, Gapellinia is clearly 

 a reversed Pentatiien^s. As noted by them : " This remarkable shell 

 is virtually a Pentameims ohlongus in which the relative convexity 

 of the valves is reversed and the reversion carried to a great extreme." 

 In this description they state that the convex pedicle valve " shows a 

 tendency to trilobation or obscure radial plication." This tendency 

 toward radial folding is likewise apparent in Pentamenis ohlongus, 

 sometimes to a marked degree. The figures of Gapellinia niira show 

 these plications, or better, undidations to be of the same sort as are 

 characteristic of Pentainerms, that is. low, broad folds. They are 

 quite distinct from the sharply defined narrow plications of Gon- 

 chidium. 



In general conformation and gross structure Gapellinia and Brook- 

 sina ai"e similar. This is to be expected. Gapellinia is essentially a 

 reversed Pentanierus, whereas Brooksina is a I'eversed Gonchidium. 

 Pentanierus and Gonchidiwn have distinctive characteristics that 

 make their separation imperative. Nevertheless in outward form 

 and gross structure the two genera are often very similar. Gapellinia 

 and Brooksina therefore as somewhat aberrant offshoots from sepa- 

 rate but closely related genetic lines are undoubtedly deserving of 

 generic separation. The features which characterize Gonchidiwni as 

 opposed to Pentanierus are equally applicable to Brooksina as com- 

 pared with Gapellinia. 



The name Brooksina is given to this remarkable genus in honor 

 of Dr. A. H. Brooks, of the United States Geological Survey. 



So far as known Brooksina is restricted to the Upper Silurian of 

 southeastern Alaska. The only fossil suggesting Brooksina that has 

 come to my notice outside southeastern Alaska is a form figured by 



