700 



ANNELIDS OF THE OLDER ROCKS. 



Nereidina, MacLeay. 



These are the most perfect in their structure of all Annelida, as they possess numerous organs 

 and have a distinct head, which is generally provided with eyes and antennae. Some of them, after 

 the manner of Serpulina, inhabit tubes, which tubes are membranaceous, and formed by a transu- 

 dation from their body ; but in general the Nereidina are naked, and they are always agile animals 

 freely moving about in search of their prey. Aristotle calls them, u %fco\67revSpaL 9a\d(rcnai ira- 

 paifKrjaLaL ra eiBei tclls 'xepaaLais" (Lib. ii. c. 121.) ; and it is true that they are wonderfully like 

 Centipedes. The fossil impressions in the Llampeter Rocks, are too indistinct to enable us to de- 

 termine very accurately the genera and species of Nereidina which there occur, more particularly 

 as the generic characters in this group depend on such minute distinctions as are afforded by a 

 study of the mouth, antennae and eyes. I shall therefore consider the impressions fig. 1. and fig. 2. 

 to belong to the 



Genus NEREITES. A genus which comes very near to Savigny's genus Lycovis in its external 

 appearance, only the segments of the body are here perhaps more slender and in proportion 

 longer than usual. 



Spec. 1. Nereites Cambrensis. Murch. n.s. 



The body of this species seems to have consisted of about 120 segments. The feet were half the 

 length of a segment of the body, and the cirri of the feet were longer than such segment. — 

 Plate 27. Fig. 1. 



Spec. 2. Nereites Sedgwickii. Murch. n.s. 



Body much more slender than that of N. Cambrensis, and apparently consisting of a greater 

 number of segments. These segments have the feet attached to them apparently inconspicu- 

 ous, although the cirri are very distinct. Plate 27- fig- 2. 

 N.B. The impression now under consideration was clearly that of an animal, as will appear by 



the figure, where the worm has evidently, before coiling, with difficulty trailed itself along in the 



mud, in a way, which any one accustomed to collect these Aymelida will at once recognise. 



Genus MYRIANITES. 



Body linear, very narrow, and formed of very numerous segments with indistinct feet and short 

 cirri. 



Spec. 1. Myrianites MacLeaii. Murch. n.s. — Plate 27. Fig. 3. 



N.B. The softness of the texture of the foregoing three species of Annelida and the perfection 

 of the impression in fig. 1. make it very remarkable, that if articulated feet existed in the Trilobites, 

 some vestiges of them, even although membranaceous, should not have come down to us more per- 

 fect than those figured by Goldfuss. (See Ann. Scienc. Nat. vol. xv. PL 2. f. 8. and pp. 665, 66^ ante.) 



Serpulina, MacLeay. 



These are sedentary animals without eyes or antennae. They live in tubes which are either a 

 natural transudation of their body, and are either membranaceous or calcareous, or their tubes are 

 semifactitious, being then composed of an agglutination of particles of sand or other small substances. 

 The calcareous nature of the tube in some Serpulina is very advantageous for their preservation, 

 and has thus enabled us to see that such animals occurred frequently in the Upper Silurian Rocks. 

 Genus SERPULITES. 



Spec. 1. Serpulites longissimus. Murch. n.s. PI. 5. f. 1. 



(See previous description of this very characteristic fossil of the Upper Ludlow Rock, p. 608.) 



