VI. Pyrgocystis. 



1915 53 



and a similar line descending from right to left, b overlapping a, and 

 so on. This spiral arrangement is not conspicuous when the plates 

 overlap closely, as in Aurivillius' fig. 11, and in those of our 

 specimens that have clear vertical columns; but it is very plain 

 when the overlap is less, though the alternation remains regular, 

 as in Aurivillius' figs. 12, 15, 16, assigned by him to other 

 species. 



The slope of the plates towards the axis of the turret appears to be 

 much as represented in Aurivillius' fig. 13, i.e. nearer the vertical 

 below, and nearer the horizontal above. 



In c the lumen at the lower end has diameters of 1*4 and l'l mm. 

 The diameter of the turret at the top of the first tier of plates is 3 2 

 and 2-9 mm. (PI. Ill, Fig. 8). This is the only specimen that shows 

 a definite lumen at the distal end. In other cases the lumen is filled 

 with plates (PI. Ill, Fig. 7), or with plates and secondary calcite 

 (PI. Ill, Fig. 9), or it may be quite covered over with plates in 

 a more horizontal position. Grinding down n at the distal end 

 displays a section with diameters about 1*3 mm. and 2*9 mm. 

 Aurivillius gives for S. sulcatum a lumen-diameter of barely 1 mm. 

 and a turret-diameter [? same specimen and level] 3*5 mm. This 

 indicates a relatively smaller lumen, but he says that the width 

 increases towards the imagined capitulum. The section which he 

 figures of S. varium (fig. 13) represents a different set of proportions; 

 here the lumen does not increase upwards, but the contrary if 

 anything, and at its lower end it is more than half the diameter 

 of the turret, at its upper end a little less than one-third. In 

 S. strobiloides the diameter of the lumen is said to be three times 

 that of the wall ; i.e. three-fifths the diameter of the turret. It is 

 very doubtful whether the width of the lumen increased upwards in 

 our Shropshire species ; in the fossils at any rate it is filled with the 

 more horizontally disposed and crowded plates, whose position 

 cannot entirely be due to post-mortem shifting (PI. Ill, Figs. 11, 

 12, 13). 



The plates, when well preserved, are covered externally with fine 

 anastomosing rugse, showing a tendency to radiate towards the 

 curved margins. Since these are developed equally on the exposed 

 and unexposed portions of the plate, they must be regarded not as 

 superficial ornament, but as an expression of stereom structure. 



The plates are frequently broken on their free margins, so that 

 either the tops are truncate or great pieces are, as it were, bitten out 

 leaving jagged points conforming to the lines of crystalline cleavage 

 (PI. Ill, Fig. 3). This frequently gives the turret sides quite 

 a peculiar appearance, as though they belonged to an entirely distinct 

 species. The tops of the plates seem to be broken in the specimen of 

 S. strobiloides shown in Aurivillius' fig. 17, which reminds one of 

 the appearances in the Minnesota fossils. 



There is no trace of any spines or spinules. But in b at the distal 

 end, which is rounded, the ordinary large turret-plates seem to be 

 covered by a number of very small plates the meaning of which 

 is obscure. 



Not a single specimen shows any trace of the oral face. 



