32 



LEPADIDiE. 



coloured in longitudinal bands. At each period of 

 growth a new and larger integument is formed under the 

 old one, which gradually disintegrates and disappears ; 

 the extreme lower point is often deserted by the corium, 

 and ceases to grow, whilst the whole upper part still 

 continues increasing in diameter: in length the chief 

 addition is made (as is clearly seen in those genera having 

 calcified scales), round the upper margin, at the base of 

 the capitulum. The surface of the membrane is either 

 naked or superficially clothed with minute, pointed, 

 articulated spines, or it is penetrated by calcified scales 

 or styles, (in Ibla alone formed of chitine,) which pass 

 through it to the corium, and are added to at their bases, 

 like the valves, at each period of growth. In Lithotrya 

 alone the scales of the peduncle are moulted together 

 with the connecting membrane. These scales on the 

 peduncle are generally placed symmetrically in whorls, 

 with each scale corresponding with the junctions of two 

 scales, both above and below. Except in Scalp ellum 

 ornatum and the fossil Loricula pulchetta, they are very 

 small compared with the valves of the capitulum. When 

 the scales are symmetrical, new ones are first formed 

 only round the summit of the peduncle, and only those 

 in the few uppermost whorls continue to grow or to be 

 added to at their bases ; afterwards membrane is depo- 

 sited under them. The shelly matter of the scales 

 resembles that of the valves, and the manner of growth 

 is the same ; tubuli generally run to and through them 

 from the corium. From the continued enlargement of 

 the membrane of the peduncle, the scales come to stand, 

 in the lower portion, some way apart. In Ibla, new 

 horny styles are formed indifferently in all parts of the 

 peduncle. In some species of Pollicipes, the calcareous 

 styles are not symmetrical or symmetrically arranged ; 

 and besides those first formed round the top of the pe- 

 duncle, there are other and larger ones formed near its 

 base. Lastly, in Lithotrya we have a row of calcareous 

 discs or an irregular, basal cup, formed in the same 



