MUSEUM OF COMPARATIVE ZOOLOGY. 21] 
farther, and place the two sub-orders of Woodward as orders,* the 
group formed by them will take the value of a sub-class, and with the 
Trilobita as a sub-class form a distinct class of the Arthopoda, as 
expressed in the foregoing classification. 
Since the above was written the memoir of Dr. Packard on 
Limulus polyphemus has been received. In addition to the descrip- 
tive portion of the investigation we find valuable comparisons made 
between the structure of Limulus and the Trilobites, and also an able 
discussion of the evidence for and against the removal of the group, of 
which Zimulus is the type, from the Crustacea, and considering it à 
class intermediate between the Crustacea and the Arachnida. 
Dr. Packard proposes *that the Merostomata and Trilobites should 
together form a sub-class of Crustacea (i. e. Branchiate Arthropods), 
standing parallel to, and as the equivalents of, all the other Crustacea, 
the two groups being parallel and equally important branches of the 
same genealogical tree.” 
While recognizing the force of Dr. Packard’s arguments, we do not 
undertake to decide between the two conflicting views as to the zo- 
ological position of the Pecilopoda. Our work has been that of the 
paleontologist, and to the zoülogist the discussion of differences that 
can only be determined by the study of the anatomy and embryology 
of living animals, is left. 
Mode of Occurrence. — The two species of Trilobites, Calymene 
senaria and Ceraurus pleurezanthemus, from which nine tenths of 
the sections were obtained, are the two most abundant forms in the 
Trenton limestone of Central and Northern New York. Their remains 
occur, usually in a fragmentary condition, in nearly every layer of the 
limestone, and range, above, into the Hudson River group, and, below, 
into the Black River limestone. Their geographical distribution is 
also very extended, as they occur in the Canadas and at nearly all the 
exposures of the Trenton group in the Northern United States, as far 
west as the Mississippi River. The Calymene is much more abundant 
at the West, but at the locality from which the specimens of Ceraurus 
were obtained for section cutting the latter far exceeds it in numbers. 
The special interest attached to the occurrence of both species at 
* The view given of the sub-order in 1877. Pamphlet published in advance of 
the 28th Report of the New York State Museum. 
+ The Anatomy, Histology, and Embryology of Limulus polyphemus. A. 8. 
Packard, Jr., M. D. Anniversary Memoirs Boston Soc. Nat. History, 1880. 
