50 



BALAN1D.E. 



Fig. 



11 



Portion of edge of basis of 

 Balcums tintinnabulum. a, a, outer 

 lamina; b, b, inner or upper la- 

 mina ; c, c, c, porose or cancellated 

 radiating septa. 



corium generally deposit transverse calcareous septa, exactly 

 as within the parietal tubes. When the basis is thick the 

 septa themselves (ccc) between the proper basal tubes, be- 

 come porose, (or rather cancellated,) 

 and they sometimes expand into a 

 very thick, cancellated layer, sepa- 

 rating the outer lamina (a) of the 

 basis from the proper basal tubes, 

 which always lie close under the 

 inner lamina [b). This structure 

 differs only slightly from that seen 

 in the parietes of Tetraclita, in 

 which the branching of the longi- 

 tudinal parietal septa, produces 

 thick walls, formed of several rows 

 of tubes or pores. With respect to peculiarities in structure 

 of the basis, Balamis Icevis offers the most remarkable case ; 

 for here, in specimens which have grown crowded together, 

 the whole interior appears sometimes to have become 

 too much elongated or too deep for the animal's body, and 

 consequently the lower part of the deeply-concave basis 

 has been filled up (PL 4, fig. 2 a) by thin, irregular, cal- 

 careous diaphragms. In elongated specimens, also, of Ba- 

 lanus balanoides, the shell sometimes appears to have grown 

 too long for the animal's body ; but in this case the mem- 

 branous basis becomes extremely convex inwards ; it still 

 reaches the basal edges of the parietes all round, but in the 

 middle it is raised high above the surface of attachment ; yet 

 sometimes threads of the cementing tissue depend from the 

 middle part to the surface of attachment. In Balamis 

 terebratm (PI. 8, fig. 2 a, 2 b), and in some species of Acasta, 

 the basis is riddled, as previously stated, by numerous, 

 minute, membrane-covered orifices. In B. declivis the 

 membranous basis is always extremely oblique, owing to 

 the rostral end of the shell being twice as high as the carina! 

 and opposite end. 



Regarding the very remarkable means by which the 

 basis of sessile Cirripedes is cemented to the surface of 

 attachment, it will be convenient to defer for a little the 

 description, on account of its necessary length. 



