MUSEUM OF COMPARATIVE ZOOLOGY. 219 
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 Branchipus and theoretically for the Trilobite. In sub- 
sequent work the proportion was found to remain ne: arly 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 Paradoxides, eighteen inches in length, 
oceurring 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 Calymene 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 
Calymene 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. Lyc. Nat. History, XI. p. 159, 1875. 
