8 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. 60. 



extreme tips of the beaks. In individuals measuring 5.0 mm. in 

 length by 7.0 mm. in breadth the ventral valve is still slightly con- 

 vex and somewhat deeper than the dorsal valve. In specimens 9.0 

 mm. in length by 12.5 mm. in breadth the convexity of the dorsal 

 valve and its relativel}^ greater size is well shown. In specimens 

 of this size the general outline of the ventral valve in side view is 

 concave. At this stage the relative proportions of the two valves 

 are much as in adult individuals except that in later stages the dorsal 

 valve becomes relatively deeper. As noted above, the largest in- 

 dividuals of the species seen measure 45.0 mm, in breadth by 36.0 

 mm. in length and 45.0 mm. in breadth by 40.0 mm. in length. The 

 average adult of the species as found in the collections gives meas- 

 urements of about o6.0 mm. in breadth by 30.0 mm. in length. 

 Throughout the series the breadth is consistently greater than the 

 length. The relative proportions change somewhat, the tendency 

 being toward a relatively greater widening in the adults. 



Brooksbia alaskensis was found in great abundance along the 

 banks of a small stream entering Davidson Inlet along the south- 

 east shore of Kosciusko Island, southeastern Alaska. Associated 

 with Brooksina alaskensis is an interesting fauna consisting of a 

 large number of species. The same horizon with approximately the 

 same fauna was found on the north shore of Heceta Island, to the 

 south. 



The rocks carrying Brooksina alaskensis are tentatively referred 

 to the Upper Silurian, as noted above. 



The type specimens are in the collections of the United States 

 National Museum, Cat. No. 68762. 



EXPLANATION OF PL.ATE. 

 Brooksina alaskensis, new species. 



Fig. 1. Young specimen (x2) siiowing preponderance of ventral valve at tliis 



stage. 

 Fig. 2. Ventral view of larger specimen. 

 Fig. 3. Youug individual (x2) showing internal structure. 

 Figs. 4, 5. Larger specimen split longitudinally to show septa, spondylium and 



crural plates. The long crural apophysis and the plication of the ventral 



septum in the anterior portion are well shown. 

 Figs. 6, 7, 8. Outlines showing progressive change in relations of septa, crural 



plates and spondylium. Figure 6 represents a cross section in the posterior 



portion of a specimen. Figure 7 represents a section about half way between 



figures 6 and 8, the latter being cut near the anterior margin of the crural 



plates. 

 Fig. 9. Lateral view of individual of medium size showing concave profile of 



ventral valve. 

 Figs. 10, 11, 12. Lateral, dorsal, and ventral views respectively of a large 



individual. 



