suBCLAss I TRILOBITA 707 



between the fixed and free cheeks furnishes another means of assisting in the 

 determination of rank. 



The pleura from the Segments of the glabella are occasionally visible, as 

 in the young of Elliptocephala, biit usually the pleura of the neck segments are 

 the first and only ones to be distingiiished on the cephalon, the others being 

 so completely coalesced as to lose all traces of their individuality. The pleura 

 of the pygidium appear soon after the earliest protaspis stage, and in some 

 genera {Sao, Dalmanües) are even more strongly marked than in the adult 

 State, and much resemble separate segments. The growth of the pygidium is 

 very considerable through the protaspis stage. At first it is less than one- 

 third the length of the dorsal shield, but by successive addition of segments it 

 soon becomes nearly one-half as long. In some genera it is completed before 

 the appearance of the free thoracic segments, all of which are added during the 

 nepionic stages. An Interpretation of these facts, to apply in valuing adult 

 characters, would indicate that a very few segments, both in the thorax and 

 pygidium, may be evidence of arrested development or suppression. On the 

 other band, the apparently unlimited multiplication of thoracic and especially of 

 abdominal segments in some genera is also to be considered as a primitive char- 

 acter expressive of an annelidan style of growth. Genera like Asaphus, Phacops, 

 etc., having a constant number of thoracic segments accompanied by other char- 

 acters of a high Order, undoubtedly represent the typical Trilobite structure. 



These analyses and correlations clearly show that there are characters 

 appearing in the adults of higher and later genera, which successively make 

 their appearance in the protaspis stage, sometimes to the exclusion or modifica- 

 tion of structures present in the more primitive larvae. Thus the larvae of 

 Dalmanites or Proetus, with their prominent eyes and glabella distinctly 

 terminated and rounded in front, have characters which do not appear in the 

 larval stages of ancient genera, but which may appear in their adult stages. 

 Evidently such modifications have been acquired by the action of the law of 

 earlier inheritance or tachygenesis, as it was called by Hyatt. 



Position in the Zoological System. — Since Trilobites have been made the sub- 

 ject of special study, they have been commonly classed with the Crustacea, and 

 placed near the Phyllopods by most observers. Quite a number of naturalists, 

 however, still divorce the Trilobites and Limuloids from the Crustacea, and 

 ally them with the Arachnids. Leaving aside the question of the homologies 

 of Limulus, it is a fact that Trilobites show the clearest evidence of primitive 

 Crustacean affinities, in their protonauplius larval form, their hypostoma and 

 metastoma, the five pairs of cephalic appendages, the slender jointed antennules, 

 the biramous character of all the other limbs, and their original phyllopodiform 

 structure. They differ from Limulus, not only in most of these respects, but 

 also in not having an operculum. From Limulus and all other Arthropods 

 they are distinguished by having Compound eyes on free cheek-pieces, which 

 apparently represent the pleura of a head segment that is otherwise lost, 

 except possibly in some forms of stalked eyes and in the cephalic neuromeres 

 of later forms. The most recent discussions as to the affinities of Trilobites 

 are to be found in the papers by Bernard, Kingsley, Woodward and Beecher, 

 where, from the facts presented, the relationships of these animals with the 

 Crustacea follow as a necessary corollary. 



As to the rank of the Trilobites in a classificatory scheme, there is also 

 much diversity of opinion. They have been long regarded as an order of 



