Tlte Life-liidonj of Spivifev hen's. 153 



beaks overarching, area high and short, and decidedl}^ shorter 

 than the greatest width of the shell, auJ the striae suddenly 

 turning in to meet the shortened liinge-line. 



Sd. The type seen in some specimens of so-called S. sulca- 

 tus, His. The typical characters of this variety are an extended 

 hinge-line. — a shorter shell, — the area low and produced later- 

 ally, the beak moderate and not overarching, — the plications 

 more than four on each side of the si'nus, and abruptly rounded 

 and distinct. I would separate those with sharp angular plica- 

 tions and prominent imbricated concentric striae, as a distinct 

 species (probably the true S. sulcatu>i, His.), Avliich present also 

 the sharp and often considerably produced hinge-line and area. 



These are the three prominent directions of variation noted 

 in the first appearance of the combination of characters which 

 marks either one of the varieties, 



Davidson's species (see Brit. Sil. Brach., i)p. 01, 92 — 98) from 

 the Wenlock, etc., are identified somewhat differently from those 

 of Hall, The ribs of S. sulcatus are not angular, as in Hall's 

 species; and Davidson's descrij)tions, as well as his figures, show 

 the close relationship between the three species called ;S'. sulca- 

 tus, His., 1. c. 91, Fig. 4-6, PI. X; S. elevatiis, Dalman, 1, c. 

 95, Figs. 7—11, PI. X; and S. crispiis. His., 1. c. 97, Figs. 

 13—15, PI. X, 



Davidson recognises the likeness of varieties of these species, 

 and appears to regard them as distinct, — rather yielding to the 

 custom of palaeontologists than on account of certain marks of 

 distinction (see 1, c, ), 



S. crispiis and ^S*. elevatus, I think may be united, and while 

 S. sulcatus may be distinct, as identified for part of the speci- 

 mens so-called in Great Britain and America, I judge that this 

 too, in part, is but one of the varieties of the typical form which 

 appears wiih much variation in Britain and America, and yet, 

 with all its variation, with well-marked "specific characters." 

 Much more might be said in regard to this point, but I lack the 

 specimens needed for examination ; and without consulting the 

 specimens themselves, we must leave the strict boundaries unde- 

 fined, and simply recognise the presence of the specific form 

 Avith all the variational peculiarities in Britain and Europe. 



In Spirifer crispus of the other side of the iltlantic, we see 



