CARADOCIAN CYSTIDEA FROM GIRVAN. 421 



Text-figs. 29-32. —Trochocystis. 



All, as shown by the millimetre scale, are about 2 '7 times natural size. All are photographed from squeezes of the 

 imprints, and these squeezes represent the actual specimens as lying with the obverse face uppermost. 



The letters and A are in each figure placed vertically above the positions of the Oral and Anal apertures (intake and 

 vent) respectively. The letters /.gr. are placed beside the food-grooves, where they are visible. 6' indicates either the 

 remains of the Stem or the position of that organ ; c.p. indicates cover- plates. 

 Figs. 29 and 30. — T. bohemica, from the Cambrian (Paradoxidian) of Skrej, Bohemia. 



This fossil is preserved in the K. Bbhmisches Museum, Prague. The sc^ueezes from which the photographs were made 

 are in the British Museum. 

 Fig. 29. — The specimen in the upper half of text-fig. 30, viewed from the oro-anal end. The food-grooves are seen passing 

 round the frame and bending down into the breach for the intake (below 0) ; that on the right hand of the observer 

 is carried over the top of the anal flooring-plate (below A) and is here curved downwards slightly ; in it are still preserved 

 several cover-plates, which are largest and most clearly seen near the intake above the letters c.p. 

 Fig. 30. — The upper specimen is that shown in text-fig. 29. The fuod-groove on the right of the intake is more clearly seen in 

 this view tlian it would have been in life, because the substance of the squeeze has been slightly distorted. The food-groove 

 on the left is visible only in the immediate neighbourhood of the intake. In the frame ten maiginals are to be distinguished, 

 and one of these is the flooring-plate of the vent region. The plates adjoining this last rise into stout prominences. The 

 floor of the thecal cavity, within the frame, appears to be formed for the most part by the plates of the reverse face, 

 seen from the inside ; but near the frame, especially to the left, are several fairly stout and slightly rugose plates of the 

 obverse face. 



The lower specimen is the original of Bakrande's " Cystidees," pi. iii. If. 23-25, which, however, represent the imprint 

 (" moule interne") and not a squeeze as here. In this photograph the food-grooves are visible only close to the intake ; but 

 in the squeeze that on the right can be traced along the edge of two marginals. The frame is constructed essentially as in the 

 upper specimen. The apparent gap, just below the prominence to the left of the vent, regarded by Bakrande (1887, p. 188) 

 as an orifice, is, I have little doubt, accidental, and its very occasional appearance seems due to the shifting of the marginal 

 that bears the said prominence. The absence of all trace of a stem may be a deceptive appearance. Regarding this squeeze 

 for the moment as an actual fossil, and comparing it with the specimen above, we shall see that the stem, if preserved, is 

 hidden under the matrix. The plates on the floor of the thecal cavity are for the most part the disarranged plates of the 

 obverse face. 



Figs. 31 and 32. — T. Barrandei,* from the Paradoxidian of Coulouma (Herault). 



Fig. 31. — Brit, Mus. E7694. The marginals near the vent and intake are slightly displaced. Near the intake the food- 

 grooves are clearly shown, and that to the right contains a few minute cover-plates. The grooves soon become very 

 shallow, but are visible in the obverse view, as shown on the left of the specimen, at f.g. The protuberance near the 

 inner end of the suture between these two marginals is in part, if not altogether, accidental. The anal floor-plate and the 

 reverse plating are seen. At the inner edge of the marginals on the left are traces of a rebate to which probably the obverse 

 integument was attached. The stem appears as partly buried iff the matrix. 



Fig. 32. — Brit. Mus. E7696. This ditt'ers from the other specimens mainly in the preservation of a large plate roofing the 

 region of the vent, and of the adjoining plates of the obverse integument ; all these are clearly rugose. The edge of 

 the anal flooring-plate is seen in natural position, and the food-groove borne by it is clearly visible. The marginal between 

 this and the intake is displaced, and shows the food-groove with cover-plates ; along the inner edge of this groove is a 

 slight rebate indented with notches where the edges of the cover-plates were hinged to it, too minute to be shown clearly 

 ou this scale. 



* Trochocystis Barrandei Munier-Chalmas & Bergeron, in J. Bergeron, 1889, p. 338, pi. 3, f . 6. Specimens in the British Museum, E 7687- 

 E7697 ; presented by J. Miquel. This name was probably overlooked by Jaekel (May 1901, p. 670) when he published the name T. occidentalis. 

 [While this was printing, I had the privilege of studying the beds at Coulouma under the guidance of Mr Miquel. A few dark iron-stained films 

 uf clear-cut outline seem to confirm the suggestion (§ 252) that broad-leaved sea-weeds grew in the same waters. Trochocystis or an allied genus 

 also occurs in the Tremadocian of H6rault ; and it is here that we may hopefully seek the intermediate forms foreshadowed in § 259. June 1913.] 



