6 HINDE, ANNELID REMAINS FROM SILURIAN OF GOTLAND. 



Jt is a curious circumstance that out of hundreds of exam. 

 ples of these detaclied jaws, I liave not been able to detect 

 a single specimen, which, from its form, can be referred to 

 the so termed under jaws in the existing representatives of 

 the order. And yet if we inay judge from the close resemblance 

 of many of the fossil forms to constituent portions of the up- 

 per jaw in the living forms, there seems good reason for sup- 

 posing that the Silurian annelids also possessed the equivalent 

 under, as well as the vipper jaw plates. In existing species, 

 the plates of the under jaw are frequently of amore calcareous 

 composition than those of the upj)er, and thus would naturallv 

 have been supposed more likely to have been preserved than 

 the mostly chitinous upper jaws; and such is really the case 

 in those examples of annelids from Solenhofen, as stated by 

 Ehlers [Palfeontogr. Bd. 17 p. 149] who remarks that »The under- 

 jaw and its cutting piece are always best preserved, probablv 

 because these contain deposits ofcalcite»; whilst the chitinous 

 vipper jaws in the same specimens have disappeared in the 

 fossilization, and are only known by the impressions which 

 have remained. In the Silurian strata on the other hand, a 

 somewhat reverse process appears to have taken place, for the 

 chitinous upper jaw plates have been perfectly preserved, 

 whilst there are no traces of the under jaws. It is true that 

 the under jaws are frequently smaller, and also from their 

 lighter tint, would be less conspicuous than the plates of the 

 upper jaws, and consequently more liable to escape notice; 

 but from the very close scrutiny which I bestowed on the 

 material in which the upper jaws were so numerous, it is vexy 

 improbable that any of the under jaws could have been pre- 

 sent in it withovit being detected. Further, a similarly con- 

 spicuous absence of the under jaw plates also ocurred in the 

 specimens from the English and Canadian Silurian, for amongst 

 these, there was but a single form Arahellites quadratus, Hinde 

 [Quart. Jour. Geol. Soc. Vol. 35 pl. 19 f. 14] which I referred 

 to a portion of the under jaw, and this dilFers so much from 

 the -normal forms of the under jaws in existing annelids, and 

 has, moreover, the same dark chitinous appearances characteri- 

 zing the distinctive upper jaw plates, that I am now disposed 

 to doubt the correctness of my former reference of it to an 

 under jaw plate. 



