154 DEVONIAN FAUNA. 



the multitudinous examples preserved in different collections, though they evidently 

 vary very greatly. Three lines of variation are apparent in them. (1) The area 

 is sometimes narrow and concave, as in the type ; and sometimes extremely broad 

 and flat, as in the form Sp. Barumensis, Salter, MS., of which examples are found 

 in which the area is much higher than wide, and the umbo hardly if at all incurved. 

 (2) The dental plates are sometimes short and sometimes extremely large and 

 massive, extending almost to the front of the ventral valve. (3) The minute 

 ornament of the shell (which is rarely preserved) is seen sometimes (in specimens 

 from Ashford Strand) to be fine transverse lines, sometimes (in specimens from 

 Ashford Strand) fine longitudinal lines, and sometimes (in specimens from Snapper 

 Quarry) coarse, concentric, irregularly arching rows of pores. It appeared to me at 

 one time that these variations might indicate specific differences, but the exami- 

 nation of foreign specimens does not seem to support this idea. 



With regard to the first point, the very numerous figures given by Gosselet 

 show very great variation in the size of the area, though perhaps hardly as great 

 as is seen in the North Devon shells. Moreover in the latter I observe that there 

 does not seem any distinct demarcation to be drawn between shells with narrow 

 areas and those with broad, and that both varieties occur of all sizes. In none of 

 our specimens have I seen any indication of a foramen. 



"With regard to the second point, the dental plates appear to be most developed 

 in large shells with broad areas, but in smaller specimens with broad areas they are 

 sometimes very short ; while Davidson has figured a small specimen with a narrow 

 area from Buclleigh Salterton in which these plates are exceedingly developed. It is 

 to be noted that Hall describes them as being inconsiderable in his Aperturati-group 

 (which includes Sp. Verneuilii) except in a few shells, which he separates from the 

 " disjunctus-type " under the name of the " Hungerfordi-type." He, however, 

 states them to be very differently developed in shells of adjacent external form. 



With regard to the third point, at first sight it appears to be impossible to 

 reconcile the three variations of minute ornament given above. Davidson states 

 that the surface is covered by numerous fine contiguous concentric lines. Gosselet 

 figures one specimen with similar concentric lines, while he figures Gyrtia 

 Murchisoniana with fine longitudinal lines. In our specimens I find that these two 

 classes of ornament certainly occur together, though where one is prominent the 

 other is almost obliterated. Few of the Continental specimens I have examined 

 show the minute characters. In specimens of Sp. Lonsdalii three coarse longi- 

 tudinal lines are seen on each rib, as originally described ; and in Murchison, 

 von Keyserling, and de Verneuil's ' Russia,' a specimen of Sp. Archiaci is figured 

 with an irregular spinous ornament. But in the British Museum is a very large 

 series of Spirifers from China, some of which are referred to Sp. Verneuilii (one 

 being so labelled in the Davidson Collection, along with a specimen exactly 



