CARADOCIAN CYSTIDEA FROM GIRVAN. 461 



and American forms, is reserved until the old species have been re-diagnosed and the 

 new ones described (§§ 551-558). 



§ 394. Diagnostic Characters of the Species. 



Outline. When the base is about at right angles to the stem, the sides up 

 to about the middle of Circlet III. approximately parallel, and the shoulders sloping 

 rapidly towards the oral pole, then the outline is subquadrate. When the base 

 is widened, and the sides as well as the shoulders have an orad slope, then the outline 

 is subtriangular. When this latter is modified by the base sloping upwards from the 

 stem to the um bones of the marginal plates in Circlet II., then it becomes trapezoid. 

 When the base and sides curve round gently to meet one another, this changes to ovate 

 or subovate ; but an ovate outline is usually combined with a lobation of the theca 

 below the stem-attachment. Lobation, being primarily a consequence of the downward 

 extension of the rectum and vent, is more marked in plate 3, and is defined by stating 

 the level on the stem which it attains ; but there may be a minor secondary lobation 

 on the opposite side of the stem, as an attempt to restore symmetry and balance. 



§ 395. Shoulder-angle. The shoulders of the theca are formed as a rule by 

 plates 12 and 14. and the slope of their outer margins towards the oral pole often is 

 almost parallel to or even coincides with the sutures 11|12 and 10|14; the angle at 

 which these slopes, if continued, would meet varies from 40° to 115°, and seems to 

 be characteristic. It bears no constant relation to the thecal ratio, though such a 

 relation may be observed in individuals of the same or closely allied species. 



§ 396. Thecal Ratio, width : height. Width means the widest part of the theca 

 in the extension-plane. Height is measured from the stem-facet to the oral pole, and 

 does not include the lobes. Thus a theca, regarded as relatively short according to this 

 measurement, might appear relatively elongate by reason of its lobes. Height is taken 

 as unity ; width expressed as a decimal fraction thereof. 



§ 397. Ratio, plate 5. Expressed in the same way. This ratio is of importance 

 because it is often impossible to obtain both measurements in a fossil theca, whereas 

 in almost every specimen the complete outline of plate 5 can be traced. Moreover, 

 without this ratio being clearly a definite function of the other, there is, as one would 

 expect, a close correspondence between the two. This is manifest when the ratios are 

 plotted as graphic curves. Of course, if each of the five plates of Circlet II. took an 

 equal share, the rate of thecal increase would be five times that of a single plate ; but 

 owing to the relative positions of the plates and the participation of 8, 9, 6, and 7 in 

 the thecal frame, the chief burden is thrown upon plate 5. Therefore, though the 

 thecal width tends to increase more rapidly than that of plate 5, the latter still serves 

 as an index to the general proportions of the theca. 



§ 398. Pectinirhombs : number. Though three must be regarded as normal for 

 the genus, rhomb 1-5 appears occasionally to be absent ; and when this absence is 

 correlated with other features, it can scarcely be passed over as an individual 

 abnormality. See P. Rugeri and P. procera (§§ 453, 493). 



