524 



Corresjjondence — Mev. 0. Fisher. 



of Trilobita and Tsopoda, we shall find they may be, in the present state of our 

 knowledge, so retained in classification. 



Pterygotus (Fossil, extinct). 



1. Eyes sessile, compound. 



2. Ocelli distinctly seen. 



3. All the limbs serring as mouth-organs, 



4. Anterior thoracic segments bearing 



branchiae or reproductive organs. 



5. Other segments destitute of any ap- 



pendnges. 



6. Thoracic segments unanchyhaed. 



7. Abdominal segments free and well 



developed. 



8. Metastoma, large. 



Limulus (Fossil, and living). 



1. Eyes sessile, compound. 



2. Ocelli distinctly seen. 



3 . All the limbs serving as mouth-organs. 



4. All the thoracic segments bearing the 



branchiae or reproductive organs. 



5. Other segments destitute of any ap- 



pendages. 



6. Thoracic segments anchylosed. 



7. Abdominal segments anchylosed and 



rudimentary. 



8. Metastoma, rudimentary. 



II. 



Trilobita (Fossil, extinct). 



1. Eyes sessile, compound. 



2. No ocelli visible. 



3. (Appendages partly oral, partly ambu- 



latory, arranged in pairs). 



4. Thoracic segments variable in number, 



from 6 even to 26, free and movable 

 (animal sometimes rolling in a ball). 



5. Abdominal somites coalesced forming 



broad caudal shield (bearing the 

 branchiae beneath). 



6. Lip-plate, well developed. 



Isopoda (Fossil, and living), 



1. Eyes sessile, compound. 



2. No ocelli visible. 



3. Appendages partly oral, partly ambu- 



latory, arranged in pairs. 



4. Thoracic segments usually seven, free 



and movable (animal sometimes 

 rolling in a ball). 



5. Abdominal somites coalesced, forming 



broad caudal shield, bearing the 

 branchiae beneath. 



6. Lip-plate, small. 



Should our further researches confirm Mr. Billings's discovery fully, we may propose 

 for the second pair of these groups a common designation (as in the case of the 

 Merostomata) ; meantime, the above may serve as representing the present state of 

 our knowledge. 



COiaiSIESIPOn^IDEIfTCIE. 



PORTLAND WOOD, ON THE COAST OF SUSSEX.— EEPLT TO 

 ME,. PERCEVAL. 



SiK, — I know the Portland beds in the Dorsetshire district pretty 

 well, and, as far as I am aware, the Tisbury Isastrcea does not occur 

 there at the present day. But, considering that there are fragmen- 

 tary patches of Portland beds fringing the coast for a considerable 

 distance in Dorsetshire, it is highly probable that, in not very distant 

 times, there was a considerable area occupied by them there ; as, 

 indeed, there probably now is beneath the Channel. The Portland 

 beds vary in character rather rapidly at places not far from one 

 another, so that the non-occurrence of a particular fossil in those 

 now visible, need not lead us to conclude that it may not have oc- 

 curred not very far off in former times. 



I would, however, recommend Mr. Perceval to obtain an accurate 

 determination of his fossil, so as to be sure of its specific identity. 



That erratics from the Portland beds have found their way into 

 Sussex, I believe to be a fact ; for I saw in 1866 a block of indubi- 

 table Portland fossil wood at Selsey, in the garden of Mrs. Pul- 

 linger. I was informed that it had been found on the beach at that 



