INTRODUCTION. 



It is well known amongst palseontologists that the hypostorna of Ihe trilobites has been 

 the particular object for research and description by a few authors, as Barkande, Noväk 

 and Brogger and some passing notes concerning it are given in the descriptive works 

 of other authors. In the following pages reference is often made to them and I may 

 here give a list of the chief works and the abbreviations with which they are cited be- 

 low in this paper. 



Angelin Palteontologia Scandinavica = Ang. 



Barkande Systéme silurien de Bohéme = Barr. 



Brögger Die silurischen Etagen 2 & 3 im Christian! agebiet = Br. 1. 



— Die Ausbildung des Hypostoms bei einigen Asaphiden — Br. II. 

 Hall, James, Palteontology of N. York, vol. I, II, VII = H. 

 Holm Trilobiter ur Dalarnes graptolitskiffer — Hm I. 



— Ostbaltische Trilobiten, Illamiden, = Hm II. 



Novåk, Studien an Hypostomen Böhmischer Trilobiten. Three different papers be- 

 tween 1884— 1886. = N. II, N. III, N. IV. 

 Salter Monograph British Trilobites. 

 Schmidt Ostbaltische Trilobiten I, II. IV. — Schm. 

 Woodward, Hem; v, Carboniferous Trilobites. Pal. Soe. 1883 — 84. 



Before proceeding further I shall give a general 



representation of the shape of the hy postonia. This 



c enigraatic part of the trilobite skeleton reseinbles as to 



its outline a heraldic shield being broad at its anterior 



margin, gradually tapering towards the posterior extre- 



mity, which generally is acuminated or, as in the Asa- 



phidaä, bifid. In a few, as Deiphon, Kemopleurides, it is 



nearly square. Its exteriör surface may consist of a 



single field or be divided transversally into two or three 



anterior pair of wings. d. posterior pair of lields, according to the presence of one or two shallow 

 »vmgs. e. anterioi groove, ,/. posterior groove. r| ,. , 



i h 



Hypostoma of /hon/rus. front and 

 side views. 



(]. latc-ral grooves h maculae. 



These are transverse, curved forwards near 





