946 MR. T. n. WITHERS ON 



whorl ; the lower half is considerably concave. Umbo of valves 

 apical. The valves are much less modified than is the case with 

 the valves composing the lower whorl in the more typical forms 

 of Sccdpellum, and consequently their position in the capitulum 

 is not so readily determinable. Seventeen of the valves are 

 symmetrical, and five asymmetrical. Of the symmetrical valves, 

 nine have an extremely concave basal margin with produced 

 lateral angles (PI. XCV. fig. 4) and probably rejDresent rostra; 

 five othei'S have a much less concave basal margin (PI. XCV. 

 fig. 8) and may I'epresent sub-carin^e. The valves just mentioned 

 are thought to be rostra and sub-carinaa, owing to the pronounced 

 concavity of the inner part of the valve neai- the base. The 

 three remaining symmetrical valves probably represent media.n 

 lateral valves (PI. XCY. fig. 6). They differ from the supposed 

 rostra and sub-carinae in the smaller concavity of the inner basal 

 pai't of the valve, and in the growth periods being formed of 

 two downward curves which meet on the median line, gradually 

 becoming more obtuse towards the basal margin. Of the asym- 

 metrical valves, two are subtriangular in shape with a recui'ved 

 acute apex, and probably represent carinal latera (PI. XCV. 

 fig. 7) ; the three others, which may be I'ostral latera (PI. XCV. 

 fig. 5), are about twice as wide as long, and are divided un- 

 equally by a fold extending from the apex to a position on the 

 basal margin about two-thirds the distance from the rostral 

 angle, the basal margin on each side being concave. 



Structure and Affinities. — While Darwin had little doubt that 

 the scutum and terguin figured by him belonged to the same 

 species, he did not feel so certain with regard to the carina, and 

 he pointed out that this valve had a close general resemblance to 

 the same valve in Pollicipes rigidus J. de C. Sowerby. A specimen 

 of T. tuhercidata in the British Museum (Natural History), 

 registered I. 14656, formerly in the Harris Collection, affords 

 valuable evidence on this point, for it consists of the carina and 

 the paired terga in position (PI. XCV. fig. 9), and proves that 

 the carina really does belong to the same species. Darwin 

 stated also that the apex of the carina did not project freely in 

 this species, but an examination of the carinae from Charing, 

 including the type, and of the numerous carinte from the Chalk 

 Marl of Cambridge, shows that the upper part of the inside of 

 the valve is much thickened to one-third of its length from the 

 apex (PI. XCV. fig. 1 c), and this portion is marked with 

 growth-lines, showing that it must have projected freely to that 

 extent. Proof of this is afforded by the specimen I. 14656 from 

 Charing (PI. XCV. fig. 9), in which the carina and terga are 

 in position, for the carina there projects freely. 



The carina, scutum, and tergum only of T, t%ibercidata were 

 known to Darwin, but the specimens from Cambridge include 

 several other valves which enable us to add much to our know- 

 ledge of the capitulum. The ornamentation of these valves is so 

 characteristic that no doubt can be entei'tained as to their 

 belonging to this sjoecies. Although there are as man}' as twenty- 



