190 Mr. T. H. Withers on some 



that the rostrum of F. rigidus is known, it is easy to see 

 that some of the valves of this species, hitherto considered to 

 be smaller and wider cariuse, are really rostra ; several 

 specimens are in the Geological Department of the British 

 Museum. A^'ith regard to P. pai'onai, it is clear to m(^, from 

 an examination of the specimen, that the valve figured by 

 Prof. G. de Alessandri (1906, Palseontogr. Ital. vol. xii. 

 pi. xiii. fig. 9) as a carina of P. paro7iai is not a carina but 

 a rostrum. A further specimen, which leaves no doubt as 

 to its being a rostrum, was among the valves given to me by 

 Count Luigi di Rovasenda ; it is particularly like that of 

 P. rigidus^ and agrees in being wider in proportion to its 

 length than is the carina. Up to the present, however, the 

 upper latus of P. pai-onai has not been found. 



There remained, then, P. elegans, Darwin, in which the 

 carina, scutum, and tergum only were known, but Dr. K. 

 Briinnich Nielsen has since figured (1912, Meddel. Dansk 

 geol. Foren. Bd. iv. p. 32, pi. ii. figs. 1-3, 11-12) a rostrum 

 and an upper latus of P. elegans similar in shape to those of 

 P.fallax, P. rigidus, and P. paronai. 



It is therefore proved that in P. fallax, P. rigidus, 

 P. elegans, and, except for the missing upper latus, in 

 P. paronai also, the capitular valves agree in number, 

 structure, and disposition. 



The most important evidence, however, in connection with 

 these species is afforded by the 15 peduncular plates that 

 were found on three different occasions with valves of 

 P. rigidus, and undoubtedly belong to that species. The 

 circumstances in which these plates were found (see p. 172) 

 justify the conclusion that in P. rigidus, and by inference in 

 P. fallax, P. elegans, and P. paronai, there were only 8 valves 

 to form the capitulum, and that the peduncle was formed of 

 plates similar to those of P. rigidus here figured. 



In support of this conclusion, it should be borne in mind 

 that only the smaller examples of carinse and rostra of 

 P. fallax have been mistaken for subcarinae and subrostra, 

 and therefore elements of a lower whorl ; no lower lateral 

 plates have ever been found to substantiate the claim that 

 that species had a lower whorl. It is also of significance 

 that among upwards of 300 valves of P. elegans, as has 

 already been pointed out, there was found not a single valve 

 of a lower whorl. 



Moreover, no valves of a lower whorl of P. paronai have 

 been found. This is the largest species of the genus, the 

 tcrga attaining nearly 1^ inches in length, and if lower 

 lateral plates had been present they would have been com- 



