182 DEVONIAN FAUNA. 



2. Genus CRANIA, Retzius, 1781. 

 1. CRANIA? RICTA, n. sp. Plate XXII, figs. 10, 10 a. 



1896. CRANIA EINGENS, Whidborne (not Hdninghaus) . Proc. Geol. Assoc., vol. xiv, 



p. 376. 



Description. Ventral valve flattish, transversely oval. Divaricator scars 

 rather small, transversely oval, slightly convex, strongly defined, crossed by 

 strong oblique ridges, and situated very near each other and near the centre of the 

 posterior margin. Occlusor scars confluent, forming apparently a long transverse 

 oval prominence, covered with transverse ridges, and situate at about the 

 posterior fourth of the median line. Ventral adjuster (?) scars very small, 

 obliquely oval, situate at the antero-lateral margins of the occlusor scars. Inner 

 surface covered by minute closely-arranged tubercles or granules. 



Size. Length about 14 mm., width about 19 mm. 



Localities. A single specimen from Pilton is in the Porter Collection. 



Remarks. I am very doubtful about the generic position of this curious 

 fossil, and only place it provisionally in this genus as its muscle-marks appear not 

 unlike those of some species of Crania. In many ways it seems to be remarkable. 

 The specimen, which is almost flat, and may, I think, be regarded pretty confidently 

 as a ventral valve, shows no signs of having been attached to any other organism. 

 The striation of the muscle-scars is very strong and acute ; the divaricator scars 

 are unusually proximate to each other, being less than their own width apart ; 

 and the width across the pair is less than one-third the width of the whole shell. 

 The surface at the centre of the occlusor scars is blurred, so that it cannot be 

 seen whether they are fully or only partially confluent. At their anterior corners 

 may be seen two much smaller and less distinct scars, which may perhaps belong 

 to ventral adjusters. The most striking feature of the fossil, however, is the 

 coarse tuberculation of its inner surface, which conveys the idea, not of being the 

 casts of pores left in a decayed shell-structure, 1 but of being the original internal 

 face of the shell. This is, perhaps, analogous to the tuberculated border of some 

 species of Crania. 



I have been unable to find the description of any species at all approaching 

 this shell ; and, though its general resemblance to some more recent Cranix makes 

 it just possible that it may be included within the limits of the genus, it is far 

 more likely that further specimens will prove the existence of generic or even 

 greater distinctions. 



1 But compare 1892, Hall arid Clarke, Pal. N. Y.,' vol. viii, pt. 1, pi. iv ir, fig. 7. 



