OF THE TRILOBITES. 17 



times even (as in Pamdod'iilcs and Ohnm) becoming complete cross furrows.* There are 

 at tlie utmost tliree such furrows at eacli side, separating lobes from the lateral margin 

 of the head, which are partly even, partly uneven, and in the latter case form the broader 

 lobes either on the posterior part [Cali/mene), or on the anterior part [PJiacops). In other 

 cases they are entirely wanting {Illa-nus), or are only indicated as slight depressions of the 

 margin of the head {AsojjJims). Next to the central head-tubercle is seen the cephalic shield, 

 which, however, generally is not quite flat, but likewise slightly arched, so that it declines 

 more or less towards the circumference, thereby forming a cavity beneath it. Of its two 

 margins the anterior is always more strongly curved than the posterior, the former frequently 

 representing a very pointed parabola or hyperbola [Isotclrs), whilst the latter only exhibits a 

 circular arch. The latter becomes deeper in proportion to the greater or lesser projection of 

 the frequently long pointed lateral angles. The margin of this cephalic shield is either 

 extended flatly {Amplius, Isoicles), or has a distinctly protuberant margin ; in the latter case 

 either having an acute angle [Calijmene) or being rounded off [Phacops). The central portion 

 of the posterior margin, at the part where it covers the first ring of the body, usually 

 projects in a thickened, swollen, and even ring-like form {Jscqj/iuis) ; the furrow-deposit, 

 however, which I shall call neck-collar {Gelenkwulsf, sulcus verticalis of Dalman), also usually 

 disappears towards both sides so rapidly, that it scarcely extends bc3'ond the middle of the 

 lateral lobes. In other numerous cases the collar extends quite as prominently in the 

 middle of the posterior margin, but is distinctly separated from the central part, as far as the 

 lateral angles, then passes round, surrounding the latter, and is continued along the entire 

 anterior margin, frequently appearing there still higher, stronger, and more distinct than at 

 the posterior margin {Calymene, Phacops). All these differences are pretty constant charac- 

 teristics of genera or groups, and therefore demand an attentive observation ; this particularly 

 has reference to the lateral impressions of the head- tubercles, since these are probably 

 not mere ornaments, but may perhaps have reference to the organization of the mouth. 

 We usually, indeed, find swellings and protuberances on those localities of the shell of 

 the Articulata where strong muscles are attached internally, and the elevations situated 

 between the transverse impressions may, therefore, probably originate from such attachments 

 of the tracheal muscles ; so that from their number we might infer the number of gills. 

 It certainly seems opposed to this conjecture that the organization of the internal part of the 

 mouth in the case of other natural groups of Articulata is generally uniform, while in the 

 Trilobites the impressions on the head are generally very different. The force of this 

 objection, however, may be diminished by assuming that the forms in which we find defective 

 impressions were characterised either by a greater thickness of the shell, or by a slighter 

 development of the muscles, so that the traces of the impressions of the muscles were 

 rendered less distinct, or were entirely effaced. And, in fact, the genera in which such 

 impressions are wanting {Illanus, Asajjhus, and some species of Phacops) seem to possess a 



* If we place confidence in this characteristic of many perfectly preserved Olenvulcs, namely, that 

 the cross furrows of tlie head-tubercle are complete, and if we may consider it as a general family 

 characteristic, several forms would belong to th;m which have hitherto only been observed in imperfect 

 specimens. According to this, we should particularly have to enumerate Trilobites Sternbergii (Table III, 

 Fig. 7), which, in point of the cephalic stnictiire, is most nearly related to Olenus scaraboeoides, and 

 Triarf/i/us Breki as both belonging to the Oleneides. 



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