BRITISH SILURIAN BRACHIOPODA. 149 



and adult conditions are essentially different from those of Bh. {Ter.) bide7itata, Hisinger, 

 as well as from those of Bh. {Ter.) diodonta of Dalman. The two last-named species 

 therefore require to be removed from the synonyms oi Bh. borealis given at p. 174 of my 

 ' Silurian Monograph.' Salter was evidently unacquainted with the true young stages of 

 Schlotheim's species when he wrote that the shell was conspicuous, even in the young 

 state, by the two raised dorsal ribs and the deep furrow in the ventral valve with its 

 single rib; and I was myself, at that period, not well acquainted with the young of the 

 shell under description, although I did not fail to say, " I am not certain whether 

 T. hidentata of Dalman (not of Sowerby) may be a distinct species or not, and therefore 

 must leave this as an open question." 



Mr. Maw's operations having placed into my hands so many specimens of the true Bit. 

 hidentata of Hisinger and Dalman, I found that this last-named shell was always a small, 

 sharply-marked species, both in the young and the adult condition, our specimens agreeing 

 in every respect with those from Sweden described and figured by both Hisinger and 

 Dalman. 



Bhynchonella diodonta is a larger shell than is the true Bh. hidentata, with a very 

 deep sinus and larger number of ribs. It is true that some adult examples of Bh. 

 borealis have but two ribs on the mesial fold, but that is the exception, and they do not 

 show the deep sinus so characteristic of either Bh. diodonta or Bh. hidentata. Bhyncho- 

 nella crispata and B. lacunosa, Sow. ('Si]. Syst.'), are certainly synonyms of Bh. 

 borealis, and the species is correctly described at p. 175 of my 'Silurian Monograph.' 

 It is a large shell, at times exceeding an inch in length and more in breadth ; it varies 

 also very much in shape, as well as in the number and size of its ribs. The young is 

 marginally triangular, depressed, the valves being very slightly convex and the fold and 

 sinus scarcely defined ; the ribs on the fold are sometimes bifurcate, as may be seen in 

 ' Sup.,' PI. X, fig. 25. With age the valves become much more convex, and the fold and 

 sinus well developed. Mr. Salter quotes Bh. borealis from the Llandovery Rocks of 

 Saugh Hill, Girvan, Ayrshire, where it is associated with Pentamerus ohlongus, Leptocoelia 

 hemisphcerica, and other species belonging to that period. 



33. Rhynchonella sub-borealis, Dav. Sil. Sup., PI. X, figs. 5, 6. 



Shell marginally transversely oval, wider than long ; dorsal valve very convex, longi- 

 tudinally divided into three almost equal portions ; fold with three angular ribs, of which 

 the central one is usually the smallest and least elevated ; lateral slopes of the fold wide 

 and flat ; lateral portions of the valves marked by about seven strong angular ribs, with 

 a tendency here and there to bifurcate as they approach their marginal extremities ; 

 hinge-line long and nearly straight ; ventral valve convex ; mesial sinus deep and broad, 



