86 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES. 



of the thorax that there seems to be no room for the heart, but it must have been located 

 beneath the sheath and above the alimentary canal. If the latter were filled with mud, and 

 the animals lay on their backs, as most of them did at death, the canal would drop down 

 into the axial lobe and the soft heart would naturally disappear and leave 110 trace of its 

 presence in the fossils. 



The Median "Ocellus" or "Dorsal Organ." 



Many trilobites, otherwise smooth, bear on the glabella a median pustule which is usually 

 referred to as a simple eye or median ocellus, but whose function can not be said to have 

 been certainly demonstrated. Ruedemann (1916, p. 127), who has recently made a careful 

 study of this problem, lists about thirty genera, members of ten families, Agnostid;e, Eodi- 

 scidae, Trinucleidae, Harpedidce, Remopleuridae, Asaphida% Illaenidre, Goldiidre, Cheiruridse, 

 and Phacopidas, in which this tubercle is present, and had he wished he might have cited 

 more, for it is of almost universal occurrence in Ordovician trilobites. 



I have not especially searched the literature for references to this median tubercle. It 

 is often mentioned by writers in descriptions of species, but apparently few have tried to 

 explain it. Beyrich (1846, p. 30) suggested that it indicated the beginning of the alimentary 

 canal. Barrande mentioned it, but if he gave any explanation, it has escaped me. McCoy 

 (Syn. Pal. Foss. 1856, p. 146) called it an ocular (?) tubercle, and that seems to have been 

 the interpretation which most writers on trilobites have assigned to it, if they suggested any 

 function at all. Beecher (1895 B, p. 309) concurred in this opinion. 



Bernard (1894, p. 422) ascribed to this tubercle, as well as to the median tubercle on 

 the nuchal segment, an excretory function, comparing it with the "dorsal organ" in Apus. 



Reed (1916, p. 174) states that it may be either the representative of the "dorsal" 

 organ of the branchiopods, or a median unpaired ocellus. 



Ruedemann (1916) has made the only real investigation of the subject. He came to 

 the conclusion that it was a parietal eye, without a crystalline lens, but corresponding to the 

 "parietal eye of other crustaceans, and especially of the phyllopods, which is a lens-shaped 

 or pear-shaped sac, usually filled with sea water." He found that above the "ocellus" the 

 test was usually thin or even absent, and in a few cases a dark line beneath seemed to out- 

 line the original form of the sac. His summary follows : 



It is claimed that most, if not all, trilobites possessed a median or parietal eye on the glabella. [In 

 proof of this assertion the following facts are stated:] 



1. A great number of species, belonging to more than thirty genera, possess a distinct tubercle on the 

 glabella. This tubercle occurs alone in many genera, otherwise smooth, as in the Asaphidae, and is hence of 

 functional importance. 



2. In certain cases, as in Cryptolithus tessellatus, distinct lenticular bodies [not lenses] were recognized ; 

 in others, as in Asaphus expansus, only a thinner, probably transparent test. Many other species show a 

 distinct pit in interior casts of the tubercle, indicating a lens-like thickening of the top of the tubercle. The 

 median eye therefore probably possessed all the different stages of development seen in other crustaceans. 



3. As in the parietal eyes of the crustaceans and the eurypterids, the tubercles are most prominent and 

 distinct in the earlier growth-stages, notably so in Isotelus gigas. 



4. The tubercle is especially well developed in the so called blind forms where the lateral eyes are 

 abortive, as in Cryptolithus (Trinucleus), Dionide, Ampyx. 



5. The tubercles always appear on the apex on the highest part of the glabella, where their visual 

 function would be most useful. 



6. The tubercle is generally situated between the lateral eyes, like the parietal eye in crustaceans and 

 eurypterids, on account of its close connection with the brain. 



7. Frequently it forms the posterior termination of a short crest, also as in certain eurypterids (Sty- 

 lonurus), indicating the direction of the nerve. 



