GENUS B A LAN US. 181 



tubes are tilled up solidly with shell. In some varieties of 

 B. crenatus and of amp hi trite, the longitudinal septa, near 

 the outer lamina, divide, thus giving rise to a very imperfect 

 row of outer short tubes. In B. vinacetis (PL 2, fig. 7 d) 

 the inner lamina is cancellated instead of being solid, which 

 is caused by the basal denticuli of the longitudinal septa 

 being simply united together by their ends and crossed 

 by transverse septa, instead of being consolidated into a 

 mass. In several species, as in B. Hameri, the walls con- 

 sist only of the outer lamina with longitudinal ribs, no 

 inner lamina having been formed ; the ribs here evidently 

 answer to the longitudinal septa in the foregoing species. 

 In B. flosculus and imperator the walls are solid, their basal 

 margins being formed of irregular, elongated points, and 

 little ridges (PI. 8, fig. 4 c), which apparently prefigure the 

 more regular longitudinal ribs or septa. In B. balanoides 

 the walls are generally either nearly smooth and solid, or 

 irregularly cancellated ; in B. cariosus (PI. 7, fig. 3 b) 

 two or three rows of short irregular tubes are formed 

 by unequally branching septa, almost as in the genus 

 Tetraclita. 



The Radii, in all the species, are constructed essentially 

 on the same plan as the parietes ; thus, in the typical forms, 

 there is an outer and inner lamina, with septa, which, 

 near the inner lamina, are furnished with denticuli on both 

 sides ; hence the radii are permeated by pores or tubes, like 

 the parietes ; but this holds good only in the first section of 

 the genus, for, in the other species, the tubes are filled up 

 quite solidly. The denticuli on the septa often occur only 

 on one side, or disappear altogether ; and, lastly, the septa 

 themselves often appear merely like little teeth, or disappear 

 altogether as in B. Humeri, or occur only near the bases of 

 the radii, as in B. amaryllis. A slight furrow in the com- 

 partment, against which each radius abuts, is generally 

 marked by the septa and their denticuli. .In regard to the 

 aim, their lateral or sutural edges are either thin and smooth, 

 or, more commonly, finely crenated or ribbed. The little 

 transverse crenations are homologous with the septa in the 

 radii and parietes. The edges of the alas are usually re- 

 ceived in a furrow. The diametric growth of the shell is 



