MUSEUM OF COMPAKATIVE ZOOLOGY. 211 



farther, and place the two sub-orders of Woodward as orders,* the 

 group formed by them will take the value of a sub-elass, 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 j on 

 Limiihis j)ohjphemi(s has been received. In addition to the descrip- 

 tive portion of the investigation we find valuable comparisons made 

 between the structure of Lhnulus and the Trilobites, and also an able 

 discussion of the evidence for and against the removal of the group, of 

 which Limidus is the t}'pe, from the Crustacea, and considering it a 

 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 Poecilopoda. Our work has been that of the 

 palaeontologist, and to the zoologist 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 Ceraxirus pleurexanthemus, 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 nearlj'- 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 publislied in advance of 

 the 28th Report of the New York State Museum. 



t The Anatomy, Ilistolof^, and Embrj'olocy of Limuhis pohjphcmus. A. S. 

 Packard, Jr., M. D. Anniversary Memoirs Boston Soc. Nat. History, 1880. 



