472 ^^^ F- A. BATHER. 



less. The outline of the theca, on the antanal face, coincides with the line connecting 

 theumbones of 9 and 14, 6 and 12, respectively; but above each of these lines there 

 projects as an epaulette the stout ridge of the very narrow pectinirhomb. 



5 453. The Pectinirhombs. One of the most striking features of this species is 

 the absence of pectinirhomb 1-5. This statement is based on the clear evidence of 

 the holotype, both the figured individual and the others preserved in specimen 25701, 

 whether these be studied in the matrix or by means of squeezes ; also on that of 

 specimens 964 and 25831, and, so far as they go, the imperfect theca 25703 and the 

 Oxford specimen. But the greater the certainty of the fact, the greater is the difficulty 

 presented by the original account. 



^ 454. Salter says : " The lower rhomb is closer upon the cordate base of the cup 

 than in any other species, so that the central area of the upper surface is conspicuously 

 large." The reasoning, and even the meaning, of the latter half of the sentence are 

 by no means clear. The main difficulty, however, is that the sentence implies the 

 presence of rhomb 1-5. Where did Salter see it ? In the list of " N. Welch Caradoc 

 Fossils," on p. 262 of the same work, an undetermined species of Pleurocystites is 

 quoted from " Ehiwlas, Bala," and this is distinguished as " with rhombs, three on 

 one side." Now, seeing that all species of this genus previously described possessed 

 " rhombs, three on one side," why was it necessary to burden a list of names with a 

 statement connoted by the generic name itself? If, however, the specimen from Blaen- 

 y-Cwm, S. Wales, taken as the holotype of P. Rugeri, had no basal rhomb, then the 

 statement was justified, and was indeed necessary to distinguish the Bala specimens 

 from P. Rugeri. It may therefore be that Salter's remark about the basal rhomb 

 was based on a specimen from Bala or elsewhere, a specimen which probably did not 

 belong to P. Rugeri at all. Unfortunately the Rhiwlas specimen cannot now be found. 



§ 455. Pectinirhombs 11-12 and 10-14 are more clearly distinguished in the anal 

 counterpart, where they are viewed from the interior ; on the antanal counterpart only 

 10-14 is well seen (text-fig. 69). The feature of both rhombs is their narrowness and 

 extension across the suture ; and this seems to have affected their structure. Rhomb 

 10-14, when the theca is viewed from the side, is seen to lie almost horizontally, but rises 

 slightly from 10 towards 14. Its outer surface in the direction of its length is slightly 

 convex. Its extreme length is 6'5 mm.; its width, 1'5 ; a ratio of more than 4:1 

 (Salter gives 3:1). The folds, which are not more than nine, in external view appear 

 to run from end to end of the pectinirhomb, but, as seen from the interior (7434), are 

 more compressed towards plate 14 and fan out towards plate 10, while alternate folds 

 disappear and the five remaining become broader. A similar arrangement obtains in 

 pectinirhomb 11-12, but here the maximum number of folds observed is eleven. Such 

 are the appearances in the imprints and squeezes, but, in the absence of the actual 

 stereom, it is difficult to understand their meaning (cf. § 510, P. quadrata). 



§ 456. The flexible integument of the Periproct occupies almost the whole of the 

 anal face, reaching to within '8 mm. of the stem, and almost, if not quite, up to the 



