MUSEUM OF COMPARATIVE ZOOLOGY. 213 



In a former paper * it is stated that 1,110 Trilobites out of a total 

 of 1,160 had been found resting on their backs, and it was argued from 

 this that that was their normal position when living, as Burmeister had 

 shown for BrancMpus and theoretically for the Trilobite. In sub- 

 sequent work the proportion was found to remain nearly the same, but 

 with the discovery of ambulatory thoracic legs the view of their living 

 in that position was necessarily abandoned. Mr. Henry Hicks writes 

 that he had observed the same position in the Primordial Trilobites of 

 Wales, the shell of the great Paradoxicles, eighteen inches in length, 

 occurring with its dorsal surface downward. He attributes it, and I 

 think correctly, to the accumulation of gases in the viscera, which, with 

 the boat-shaped shell, would cause the animal to turn over on the slight- 

 est motion in the water, and it would there remain to be buried beneath 

 the next deposit of sediment. 



A little dark argillaceous shale next succeeds, above the prolific 

 Trilobite layer, and forms a parting between the latter and a layer of 

 limestone six inches thick that is very much like the layer below in 

 color and texture. From it several hundred very perfect Asaphi 

 have been taken ; but, with the exception of two small enrolled speci- 

 mens, they have not shown the presence of the appendages sought for. 

 Smooth, fine-grained, dark gray and bluish-gray limestones, in layers 

 of from one to four inches in thickness, succeed in the next three feet 

 above. Trilobites abound in nearly every layer, and upwards of fifty 

 species of fossils, in a very perfect state of preservation, occur in the 

 same beds. The conditions, however, do not appear to have been 

 favorable for the preservation of the viscera and appendages, and 

 the most perfect enrolled specimens of Cahjmene have nothing but the 

 clear rock within the dorsal shell. On the lower surface of the pro- 

 lific layer and in its interior many specimens of Acidaspus Trentonen- 

 sis were found. Owing to their small size, five to ten mm. in length, 

 they were not of much use, although some of their appendages were 

 frequently found in a somewhat entire condition. The specimens of 

 Cahjmene and Ceraurus averaged from thirty to forty mm. in length. 

 A full series of the latter shows individuals from three up to fifty mm. 

 None were ever seen showing any metamorphoses in the young, from 

 this layer, although Asaphus platycephalus was found with but three 

 segments in the thorax on a layer above. 



As far as we now know, the occurrence of the Trilobites in the pro- 

 lific layer is an exceptional one, as none have been reported from any 



* Ann. Lye. Nat. History, XI. p. 159, 1875. 



