On the Brain o/Limulus polyphemus. 29 



horizontal distribution. A form belonging to this genus has 

 been met with deep within the Arenig forcnation ; and its most 

 recent species marks the highest known zone of the Bala 

 rocks. 



In the Llandeilo rocks of Wales forms of Dicellograptus 

 are common ; but the genus does not appear to attain its 

 maximum until we reach the higher zones of the Glenkiln 

 shales. In the succeeding Lower Hartfell beds examples 

 occur in countless multitudes on several horizons, but the 

 species are fewer. In the highest Birkhill only two species 

 are known. 



The most conspicuous species is D. sextans, Hall, which 

 marks the Upper Llandeilo and Glenkiln shales in Wales and 

 Scotland and their equivalents in Scandinavia and in North 

 America. D. complanatus, Lapw v and D. anceps are the 

 characteristic fossils of the Upper Hartfell beds, as Dicello- 

 graptus Forchhammeri, Geinitz, is of the Lower Hartfell. 



Dicranograptus. — The genus Dicranograptus (which is, 

 morphologically, merely a Dicellograptus whose branches are 

 conjoined proximally for a portion of their length) is as yet 

 unknown in the Upper Arenig. Its oldest known species is 

 of Llandeilo age. It attains its specific maximum in the 

 Glenkiln zones, only two British species surviving into the 

 Lower Hartfell. In the Upper Caradoc subdivision the genus 

 is as yet unknown. 



The most conspicuous species is D. ramosus, Hall, an 

 Upper Llandeilo and Glenkiln fossil in Britain. Like its 

 associate Coenograptus gracilis, Hall, its horizontal distribu- 

 tion appears to have been almost world-wide. 



[To be continued.] 



VL — On the Internal Structure of the Brain of Limulus 

 polyphemus. By A. S. Packard, Jun. 



Several years ago I attempted to study the brain of the 

 horse-shoe crab (Limulus polyphemus), and had it sliced into 

 a large number of sections. Owing to interruptions these 

 sections, made from unstained alcoholic specimens, were not 

 examined. During the past winter I have been able, with 

 the aid of Mr. N. N. Mason of Providence, to take up the 

 study afresh. Mr. Mason has kindly made sections, both 

 transverse and horizontal, stained with osmic acid, — also sec- 

 tions of the brain and supraoesophageal ganglion of the 



