expanded by Mr. "Woodward *, may or may not meet with general acceptance, it is plain 

 that Limulus has closer affinities v/ithPteri/gotus and other Eurypteroid Crustacea of palae- 

 ozoic age than with the Tribolites. My aim, therefore, in finally selecting, in 1843, Limulus 

 for anatomical research reflecting light on the organization of palaeozoic Crustacea, is 

 equally fulfilled by the subsequent discoveries of Agassiz t, M'Coy J, Hall §, Niesz- 

 kowski||, Salter^, Huxley^, and Woodward **, of extinct Crustacea of as high anti- 

 quity as the Tribolites, and more closely represented by Limulus. I doubt not therefore 

 that the following illustrations of the structure of their lingering representative will be 

 acceptable to Palteontologists as well as to Comparative Anatomists. 



The contributions to the anatomy of Limulus previously made will be noticed in con- 

 nexion with the sections to which they belong ft. 



§ 2. External Characters. — My remarks on this head need be few, and bear mainly on 

 the intelligibility of the anatomical details. 



* ' Reports and Proceedings of the British Association, Edinhiirgh, August 1871.' Mr. Woodward exemplifies 

 his views by the following concise parallel : — 



" Order MEROSTOMATA, Dana. 

 " Suborder ETJKrPTEKiDA. I. " Suborder Xiphosuka. 



" Ex. Ptenjgotus (Fossil, extinct). 

 ' 1. Eyes sessile, compound. 



2. Ocelli distinctly seen. 



3. AU the Umbs serving as mouth-organs. 



4. Anterior thoracic segments bearing branchias or 



reproductive organs. 



5. Other segments destitute of any appendages. 



6. Thoracic segments unanehylosed. 



7. Abdominal segments free and ivdl-deve- 



loped. 



8. Metastoma large. 



" Ex. Limulus (Fossil, and living). 

 " 1. Eyes sessile, compound. 



2. OceUi distietly seen. 



3. All the limbs serving as mouth-organs. 



4. All the thoracic segments bearing branchiae or 



reproductive organs. 



5. Other segments destitute of any appendages. 



6. Thoracic segments anchylosed. 



7. Abdominal segments anchylosed and radi- 



mentary. 



8. Metastoma rudimentary." 



t ' Monographie des Poissons Fossiles du Tieux Gres Rouge,' &c. 4to, p. xix. 1844. 



X ' Contributions to British Palseontology,' Cambridge, Svo, 1849. 



§ Prof. James Hall, LL.D. ' Natural History of New York,' part vi. Palaeontology, vol. iii. 4to, 1859. 



11 Archiv fiir die Naturkunde Liv-, Ehst- und Kurlands, erste Ser. vol. ii. 1859. 



^ " On some new Crustacea from the Uppermost Silurian Rocks " (Salter), " Observations on the Structure and 

 Affinities of Himaritojitertts " (Huxley), Quart. Journ. Geol. Soc. Lond. vol. xii. 1856; "On the Anatomy and Affi- 

 nities of Pteryyotus" (Huxley & Salter, in Monograh 1, 'Memoirs of the Geological Survey of the United Kingdom,' 

 Svo, 1859). 



** " On Eurypterus lanceolatus," Geol. Mag. vol. i. 1864. British Association Reports, 1864. Quart. Journ. of 

 the Geol. Soc. Lond. vol. sxi. (1865). 



tt Straus Diirckheim, in his ' Anatomic Compare'e des Arachnides,' June 1829, pointed out some particulars of 

 structure, the '-sternum interieur," e. </., in which Limidus resembled the spiders. This led LatreiUe to designate the 

 Linndi " Crustaces-arachnides " (Dictionnaire d'Histoire Nat. Art. ' Entomologie, Litnuli'). Other resemblances to 

 Arachnides in organization will be pointed out in the present memoir. Whence I infer that Limulus and the extinct 

 members of the order Merostomata exemplify a more generalized condition of condylopod organization, from which 

 the Arachnida, quitting the waters, may have diverged as a special branch of air-breathers. 



b2 



