OLENELI.US AND OTHER GENERA OF MESONACID^ 24 1 



i have not been able to find maculre showing any definite structure 

 on the hypostoma of any species of the Mesonacidse. I see, however, 

 no a priori reason why such structures should not be present. 



From the structure and probable habits of the trilobite, as a mud- 

 burrowing animal more or less allied to Limnlus, it does not at first 

 appear what special purpose was subserved by having visual organs 

 on the h3^postoma. While thinking of this, I was led to revert to 

 observations that I made when collecting trilobites showing ventral 

 appendages. These notes [Walcott, 1875, p. 159] state that of 1,160 

 specimens of Ccratirus noted on the under surface of a thin layer of 

 limestone, 1,110 were lying on their backs when buried in the sedi- 

 ment and but 50 presented the dorsal surface upward. Prof. Alex- 

 ander Agassiz in describing the habits of young Limiihis says [1878, 

 PP- 75-76] : 



Mr. C. D. Walcott has called attention to the fact that when collecting fossils 

 he finds large numbers of trilobites on their back ^ ; from this he argues that 

 they died in their natural position, and that when living they probably swam 

 on their backs. He mentions, in support of his view, the well-known fact that 

 very young Limidus and other Crustacea frequently swim in that position. I 

 have for several summers kept young horse-shoe crabs in my jars, and have 

 noticed that besides thus often swimming on their backs, they will remain in a 

 similar position for hours, perfectly quiet, on the bottom of the jars where they 

 are kept. When they cast their skin it invariably keeps the same attitude on the 

 bottom of the jar. It is not an uncommon thing to find on beaches, where 

 Linndus is common, hundreds of skins thrown up and left dry by the tide, 

 the greater part of which are turned on their backs. An additional point to be 

 brought forward to show that the trilobites probably pass the greater part of 

 their life on their back, and die in that attitude, is that the young Limuhis 

 generally feed while turned on their back ; moving at an angle with the bottom, 

 the hind extremity raised, they throw out their feet beyond the anterior edge of 

 carapace, browsing, as it were, upon what they find in their road, and washing 

 away what they do not need by means of a powerful current produced by their 

 abdominal appendages. 



My object in calling attention to the above facts in relation to the 

 habit of trilobites and LiuinJus is to suggest that in all probability 

 the eyes of the hypostoma were of service when the trilobite was 

 lying on its back on the sand or mud, and it was on this account that 

 they were thus developed. It is highly probable that the adult trilo- 

 bite crawled about the bottom and did not swim freely in the water 

 to the extent that it would be necessary for it to be able to see the 

 bottom. Its habits must have been very much like those of Limidus 

 when in search of food. That the trilobite burrowed and pushed 



'Ann. Lyceum Nat. Hist. New York, Vol. ir, 1875, pp. 155-159. 



