5:58 ON FOSSIL CIRRIPEDES. 



carinse are easily distinguished, for whilst in S. arcuatum the 

 tectum is flatly-arched transversely, and the intraparietes are 

 bent inwards almost at right angles, the tectum in S. soliduhom 

 is strongly convex, and the intraparietes join to form a prominent 

 crest. Darwin considered the scutum of /S. solidtdum to be like 

 that of *S'. arcuat'U7n, with the exception of the longitudinal ridges 

 being proportionally broader and further apart, closely resembling 

 those in the carina of *S'. solichdum. 



Unfortunately the only known complete capitulum from the 

 Cretaceous rocks with which that of S. arcuatum can be compared 

 is that of the Uj)per Senonian species S. fossula. In the relative 

 positions of the valves both species are alike, but in the structure 

 of the carina and scutum there are important differences. The 

 intraparietes of the carina of S. arcuatum are sharply bent 

 inwards, the upper part of the valve is solid and must have 

 projected freely to some considerable extent. The carina of 

 S. fossula, on the contrary, projected freely but little, and the 

 intraparietes form a thin wall on each side of the carina. In 

 *S'. arcuati'jn the tergo-lateral angle of the scutum is situated 

 much further from the apex than in S. fossula, and in this 

 respect is further removed from the more advanced scuta which 

 have the tergo-lateral angle almost in line with the apex, above 

 which the valve is added to, the umbo consequently being sub- 

 central. The valves of *S'. arcuatum are longitudinally ridged, 

 while those of S. fossula are smooth. 



Affinities of the Species mentioned. 



S. arcuattcm is no doubt an ancesti^al form of a group of almost 

 exclusively deep-sea species, which Dr. P. P. C. Hoek* has 

 separated as a sub-genus under the name ArcoscalpeUum. 

 /S. trilineatum, S. accumidattmi, S. comptum, S. maxim^mi var. 

 cylindraceum, and *S'. solidtdum, which appear to be related to 

 iS. arcuatum, possibly belong to the same gi'oup, but we know too 

 little of these species to say much about them. /S. simplex 

 probably does not belong here. 



The species embraced by the sub-genus ArcoscalpeUum have 

 no sub-cainna, and it is impossible at present to say whether 

 ^S*. a7'cuaium had a sub-carina or not. The Senonian species 

 jS. fossida also comes nearest to the subgenus ArcoscalpeUum, but 

 this species is said by Ed. Hebei't t to have a sub-carina, and 

 in view of this it is possible that the Albian S. arcuatwm also 

 may have had one. Assuming this to be the case, we have two 

 forntis agreeing in all essential characters with the forms of 

 ArcoscalpeUum., except that they (possibly) have a sub-carina. 

 Moreover, the two species differ in the form of the carina, S. arcua- 

 tutn having the intraparietes bent inwards at right angles and 



* P. P. 0. Hoek, Oct. 1907, Siboga-Expeditie, Cimpedia Pedunculata, p. 59. 

 f Ed. H<5bevt, 1855, M^m. Soc. Geol. France, 2" ser. vol. v. p. 356, pi. xxviii. fig. 1 

 (S. gallicmn — S. fossula). 



