46 b \ la nidi:. 



pores. The lubes become tilled up solidly much more 

 commonly than do the parietal tubes; and the inner 

 lamina, in such cases, is hardly distinct from the outer 

 lamina. The denticuli often fail, or arc present only on 

 the lower sides of the septa; and very frequently the edge 



of the radius can only be said to be cre- 

 Fig. 10. nated. Notwithstanding these frequent 

 anomalies, if a series of species and genera 

 be taken, it is certain that there is, as 

 might have been expected, a close rela- 

 tionship in internal structure, between the 

 ■•_{ radii and the parietes. The edge of the 

 radius is received in a slight furrow 



b a o 



, (generally marked like a seal, with the 



Edge of the radius of ;° . *> . ' . 



Baianus tintinnabuium. impression oi the deiiticulatcd septa) in 



a, outer lamina; A, inner , L , . l , 



lamina; r, denticulated the opposed compartment : sometimes the 



• septa, uniting the two IX , t i p ji •• c 



laminse. outer edge or lid ot the recipient furrow, 



is so broad as to give the false appearance 

 of a radius having been developed, at least in the lower 

 part of the shell, on both sides of the suture. A crest of 

 coriuin runs into each suture between the edge of the 

 radius ano 1 the furrow in the opposed compartment; and 

 when the radius is permeated by pores (as in woodcut 10), 

 threads of corium branch off this crest, and enter the pores. 

 In the lower part of the shell, these crests of corium project 

 from the corium forming and surrounding the sack ; but in 

 the upper part of the shell, above the opercular membrane, 

 and therefore above the sack, the corium is produced up 

 each line of suture as a separate ribbon. In proportion as 

 these ribbons extend more or less near to the summit of 

 the shell, so do the edges of the radii continue to be added 

 to, to a greater or less height from the basis ; and con- 

 sequently their summits become less or more oblique. 



Peculiarities in t/ie Structure of the lladii. — In some of 

 the species of Tetraclita, in which genus the walls consist of 

 several rows of tubes, the radii are likewise perforated by 

 several rows; and in some of the other species (PI. 10, 

 fig. 1 //), the edge, or disarticulated surface of the radius, 

 is marked by irregularly branching ridges ; and these 

 evidently correspond with the branching septa or ridges of 



