VI. Pyrgocystis. 



1915 51 



Material. — The fourteen specimens presented to the British Museum 

 by Sir Arthur Anstice are registered E 16233 to E 16246, but may for 

 convenience of reference be denoted by the letters a to n. They are 

 all turrets in various degrees of preservation. Specimen c [E 16235] 

 is taken as holotype. 



Hori%on and Locality. — All are from the Wenlock Shale of Jig 

 House Bank, or, in the local vernacular, " Jiggers Bank." This, says 

 Sir A. Anstice in Hit., "is a road that runs from Coalbrookdale up the 

 right hand side of a steep dingle called Lloyd Brook dingle (the main 

 road from Coalbrookdale to Wellington runs up this hill) . . . the 

 road runs up the side of a steep bank, and there is a place called the 

 Loam or Lum hole in the lower part of the dingle." 



Detailed Description. — These fossils closely resemble the u Scalpellum 

 sulcatum " of Aurivillius, with whose description the following may 

 be compared. 



The Turret is slightly curved and increases gradually in diameter 

 from below upwards, frequently expanding rather suddenly at its 

 adoral end, where the plates appear more closely packed. 



The plates are arranged in eight columns, which may (as in c, PI. Ill, 

 Fig. 4) be quite distinct with grooves between, but which may be 

 partly merged so that it is hard to say whether the number is 7 or 

 8 (e.g. a, b,f, k ; PL III, Fig. 3), or may be clearly not more than 

 7 (e.g. d, e; PL III, Fig. 5). The vertical seriation is most plain 

 in the middle half of the turret ; towards the adoral end there is 

 nearly always an irregularity connected with the expansion of the 

 turret, which is usually more on one side and affects the packing of 

 the plates ; towards the lower end, as the turret contracts, the 

 columns inosculate and become reduced in number, so that there 

 may be only 4 or 5 at the base, increasing to 7 or 8 above 

 (e.g. m\ PL III, Figs. 6, 10). When the irregularity of the 

 upper expansion has come in before the increasing plates of the lower 

 end have taken their positions in definite columns, then the arrange- 

 ment appears quite irregular (e.g./). In those regions of the turret, 

 or in those individuals, in which the complete arrangement in eight 

 distinct columns has not been attained, the inosculation of the plates 

 frequently appears on the surface as a column of smaller plates 

 between two columns of larger ones; one result is the absence of 

 a groove between the columns and a more even rounding of the 

 surface, as described by Aurivillius for his S. varium and 

 S. strobiloides. 



The shape of the visible portion of each plate varies with the 

 extent to which it is exposed, and with the greater or less distinctness 

 of the columns. The shape of the whole plate is given in PL III, 

 Figs. 14, 15 ; and the only variation of this seems to be in the greater 

 or less relative width, producing a more obtuse or more acute angle. 

 The sides of the upper half of the plate form a somewhat parabolic curve, 

 the whole of which may be exposed ; but however little be exposed 

 it can never be likened to the segment of a circle. In the lower half 

 of the plate the sides become more straight and vertical, or even 

 converge slightly. Of course it is only at the distal end of the turret 

 that the plates are ever fully exposed, and it is probable that the 



