] 02 Frederick Cluqyman : 



43, 44. Leptaeva c/epresso, Sowerby, 1839, in Murchison's Silurian 

 System, pp. 623. 636, pi. xii., fig. 2. Strophomena rliomhoidalis, 

 Wilckens sp., Davidson, 1871, Mon. Sil. Brach (Pal. Soc), pt. vii.. 

 pt. vii., No. 4, p. 281, pi. xxxix., figs. 1-21 ; pi. xliv.. fig. 1. Leptaena 

 (Leptagonia) rhonihoidalis, Wilckens sp., McCoy, 1877, Prod. Pal. 

 Vict., Dec. v., p. 19, pi. xlvi.. fig. 1. 



Observafio?is. — As McCoy has remarked, the Victorian specimen* 

 of L. rhomhoidaliff show no difference from the European examples, 

 and exhibit the same variable characters. The species is a very 

 abundant fossil in certain of the mudstone facies of the newer or 

 Yeringian division of the Victorian Silurian, some fossiliferou^ 

 blocks being quite crowded with their remains to the exclusion of 

 other shells. Three very interesting examples of this species having 

 an important bearing on the phylogenetic relationship of this group, 

 were discovered in the mudstone at Loyola, near Mansfield, by Mr. 

 Geo. SAveet, F.G.S., and these are described below. 



JS'ote on (I specimen of Leptaena rhomhoidalis showing striate 

 devticulafions on the cardinal area. (Plate X., fig. 3.) 



Hitherto the hinge-line of Leptaena depressa has been held to 

 have a non-denticulate character. Amongst the well-preserved casts 

 of this shell in the Sweet collection, however, there is one specimen 

 in the n^anic stage, ^ 13.75 mm. wide, showing the cardinal line, 

 especially on the portions nearest to the cardinal process of the 

 opposed biachial valve, to have fine but well-marked linear ridges 

 normal to the hinge margin. 



In Hall and Clarke's summary of characters in the genus 

 Leptaena"^ those authors say " Cardinal area narrow, slightly wider 

 on the pedicle valve, not denticulate." It is conceivable that these 

 denticulae now recorded may be only partially calcified as a rule, 

 and thus similar to other structures only occasionally seen in this 

 genus, such as the spiral brachia, and the striated muscular areas, 

 and that the process of fossilisation more often tends to their disap- 

 pearance than to their preservation. Another reason to be assigned 

 for this denticulate structure hitherto escaping observation is that 

 tlie usual condition of the matrix is more or less calcareous, and this 

 tends towards a secondary mineralisation of the fossil body; whereas 

 the mudstone of th^ Victorian Silurian lends itself admirably to 

 the preservation of delicate organic structures such as this. In 

 fact, the fine, oozey mud which was laid down in those times was to 



1 That ia, a young; form developing;' the adult characters of the bhell. 



2 Pal. N. York, vol. viii., Pal. Brachiopoda, pt. i., 1892, p. 277. 



