MONOGRAPTUS. 



399 



Figs. 260 aand h. — Monniirdptus chimxra 

 (Barrande). 



apertnral margin, having a length of 2 mm., four times as long as wide, and 

 overlapping one-half their extent ; average inclination high — 40°-50°. 

 Description. — In the general form and characters of the polyparj as a whole 

 apart from the spines, Mono/j. rliininrn is somewhat like M. mJoinis, l)ut it is 

 typically shorter and broader. 



The thecas are very distinctive and present different appearances according to 

 the direction and amonnt of compression. When the theca3 are shown in profile 

 the spine has a curious position, originating, as it Avere, from the angle of a cusp 



situated in the middle of the apertural margin 

 (Fig. 2GG a). In other cases, however, in which 

 the thecse seem to narrow somewhat in the apertural 

 region, the spine appears to be in a more usual 

 position, and to come off where the ventral wall 

 meets the apertural edge. The spines of the 

 jiroximal thecse appear to be always in this latter 

 position, and it is possible that these thecae, which 

 certainly show signs of ogee curvature of their 

 walls and retroversion of their thecal apertures, are 

 different in shape from those of the more distal 

 portions of the polypary. The appearance of the 

 mature theca3 suggests that they were more or less 

 hexagonal in section ; indeed, in one specimen, 

 a h preserved so that the apertures face the observer, 



n. Complete specimen, profile view; in quc or two of the thecsB are definitely hexagonal 



relief. Walsall, Staffs; Lower . ... 



Ludlow Shales. Coll. Lapworth. (Fig. 26G h) . The apparent anomalies in the posi- 



h. Specimen, snb-ventral view, showing . . iiii i-iii- 



form of thecal apertures. Elton tiou of the spiucs w^ould tlius be explained by their 



Lane, Ludlow ; Lower Ludlow . ff p c , i i i i j i 



Shales. Coll. Wood. comiug oft ii'oni oue 01 tlic augles iiiado by the 



walls of the hexagonal prism with the apertural 

 margin. The various positions assumed by the spines under different conditions 

 of preservation may be seen from the figures. The spines are commonly stout 

 and do not exceed "6 mm. in length. 



The sicula has a length of nearly 2 mm., and extends to midway between the 

 apertures of th. 2 and th. 3. 



Affinities. — The spined character of the thecae separates Monog. cldmsera from 

 all other forms of the M. coJonns group except its own variety Saliveyi and M. 

 leintwardinensis. From the former of these it may be readily distinguished by 

 its shorter, stouter spines, and longer and more curved polypary. In M. 

 leinhvardinensis the spines are mesial in position and not apertural, except in the 

 proximal portion, where the thecae agree closely in form with those of il/. cltimasra. 



Horizon and Localities. — Lower Ludlow Shales (zones of M. Nilssovi and 

 3f. scanict(s). 



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