OF TIIH TRILOBITES. 27 



passing over into the anterior angle of the arched margin of the h)he. I doubt very much 

 whether the presence of this furrow on the lateral lobes is of any material importance in the 

 organization of the Triiobites in ^Yhieh it occurs, for if this were the case, it could not be 

 entirely wanting in some genera, for instance, in Ilhcnm ; and I rather consider it as a 

 secondary matter, caused by the presence of the transverse furrow on the central rings 

 themselves, and thus continued to the lateral lobes. This view can be supported by the 

 structure of Illaiius, in which the transverse furrow is as much wanting on the central 

 principal ring as on the lateral lobes, and which therefore have a much flatter, and more 

 uniformly arched back than the other species, the back rings being always individually 

 very strongly arched. The organization of the abdomen of the Macmra furnishes among 

 the living Crustacea an exact counterpart to the usual Trilobitc structure with furrowed 

 rings ; while the organization of the thorax of the Amph'qwdcs and Iwpode-s, on the other 

 hand, represents the form existing in lUcenus. Both modes of formation, however, admit 

 the power of rolling up, both by the Triiobites and by their living analogues referred to. 



SECTION XI. 



The number of rings, of which the thorax consists, is a circumstance of great import- 

 ance. The number may be readily ascertained in those genera which have a large caudal 

 shield, but with greater difficulty in those where the body terminates in a very small shield, 

 in which only four rings are contained. The question arises here, whether the thorax 

 can really be assumed as extending to this shield, or whether, judging from the analogy 

 of living forms, a portion of the rings before the terminating shield does not belong to the 

 abdomen, the real boundary of the latter being determined by the position of the sexual 

 opening, as in Apis. Nothing of course can be decided in this respect, owing to the absence 

 of all soft parts ; and we have therefore no alternative but to consider the thorax in the 

 Triiobites as extending to the simple caudal shield, and the rings contained in the latter 

 as the abdomen. 



Assuming then this to be the case, we find a very great difference in the number of the 

 thoracic rings. The smallest number appears to be five ;* Sarsf at least asserts that he saw 

 no more in Ampyx ro.strafi/s ; whilst, according to Dalman, Ampi/x nasufii« possesses six rings ; 

 and since I am not aware of any authenticated case in which different numerical proportions 

 of the rings occur in the same genus, I must assume that the first number is incorrect. The 

 latter number is also found in Crijptoliihts, Green ; and Trinucleus, Murch. I have hitherto 

 nowhere been able to perceive seven distinct rings ; and although this number is stated by 

 some authors as existing in Of/i/yin, the number of eight stated by others seems to con- 

 tradict the correctness of this calculation. Eight articulations are possessed by all species 



* Dr. Beyrich, in his 'Treatise on some Bohemian Triiobites' (Berlin, 1815, 4to), describes a 

 perfectly preserved specimen of Battus integer with two body rings. This number therefore was the 

 smallest. 



t ' Isis,' 1835, p. 335. 



