274 A EEYIEW OF THE TRILOIUTES. 



are provided with eyes ; whilst amongst others, as stated above, these 

 organs become obliterated by age. 



The buckler of the trilobite does not terminate immediately at the 

 upper margin or sides, but bends over as in the limulus or apus, and 

 thus forms the margin of an under shield. Directly beneath its ter- 

 mination on this under side of the head, and exactly facing the gla- 

 bella, is situated a peculiarly shaped organ, called from its general 

 characters and presumed function, the hypostoma or labrum. With 

 the exception of a second piece, the epistoma, found only in a few 

 rare examples, it constitutes all that is known respecting the mouth- 

 organs. In its general form, the labrum somewhat resembles a 

 pointed or rounded glabella, with its attached base, placed in a revers- 

 ed posilion, or with the narrower end downwards. From its resem- 

 blance to the labrum of the apus, it has been inferred that these 

 creatures were carnivorous : a view sustained by other considerations. 



3. The body or thorax of the trilobite is made up of a number of 

 separate rings or segments. By the two longitudinal furrows already 

 mentioned, each segment is divided into three parts : the middle 

 part, called the rachis or axis, and the side3 or pleurcs. It is still 

 uncertain whether the pleura? form a continuous portion of the axis., 

 or whether they are united to it by suture. Basing our observations 

 on those species which have the longitudinal furrows but slightly 

 developed, we might naturally infer the former. In most specimens, 

 if not in all, the shell is certainly continuous. Single, disjointed 

 segments are constantly met with ; their three-curved outline is one 

 of the most common markings on the weathered surface of trilobitic 

 rocks. The central rings are sometimes furnished with short spines. 

 The pleura? also frequently terminate in spines ; and they are either 

 grooved in the direction of their length — id est, from the axis out- 

 wards — or otherwise raised in the same direction into a narrow plait 

 or band. The former modification constitutes Babeande's Typede 

 laplevre a sillon: the latter his Type de la plevre a bourrelet. The 

 character in question is brought prominently forward by B abbas de * 

 as a classification element, and Pictet has also adopted it in the last 

 edition of his " Paleeontologie ;" but its employment as a leading 

 character, appears to me, for reasons stated in the sequel, to be open 

 to many objections. The well-known power of rolling themselves 

 up into a ball, possessed to a certain extent by probably all trilobites, 

 and by many in an eminent degree, was chiefly due to the mobility 

 of these thoracic segments. Further reference will be made, how- 



» See the Appendix to this paper, at the close of Part II. 



