14 OLD RED SANDSTONE FISHES. 



^ VI. Distribution of Cephalaspida. — There is not enough known with regard to these 

 Fishes to warrant any conclusions as to their distribution in time or space. The facts are 

 these : first, as to time — Cephalaspids occur in the Lower Ludlow beds, in the Upper Ludlow 

 beds, in the Downton Sandstone, in beds called Upper Silurian in Russia and Gallicia, in 

 the Ludlow Bone-bed, in the Passage-beds, and in that lowest portion of the Old Red 

 Sandstone, which some geologists think, ought to be called Silurian. 



As to distribution in space : they have been found in the West of England, in the 

 East of Scotland, in Russia at Rootsikiille, in Gallicia, and in the Eifel. There is reason to 

 hope that they may be found in North America, and other localities where Silurio-Devonian 

 beds occur. 



There is not satisfactory evidence that the same species of Cephalaspida occur in any 

 two of the above-named localities. The species of the West of England appear to be 

 distinct from those of Scotland ; and even in the Cornstones of Herefordshire and Wor- 

 cestershire the species appear to have the most limited range and definite horizons. 



§ VII. 8i/nopsis of the Divisions of the Cephalaspida. 



Familia vel Sub-ordo Cephalaspida, Huxley. 



Pisces sine ossibus internis ; caput scuto calcareo magno simplice vel composito 

 armatum : truncus parvus, caudiformis, lepidibus rhombicis magnitudine variantibus 

 ornatus. 



Sectio A. — Heterostraci. 



Scuti materia sine "lacunis" osseis, intime sinibus polygonalibus excavata, superficie 

 striis vel liris ornata. 



Genus 1. Scaphaspis. — Scutum simplex ovale. 



Genus 2. Cyathaspis. — Scutum in quatuor partes divisum, ovale. 



Genus 3. Pteraspis. — Scutum in septem partes divisum, sagittiforme. 



Sectio B. OSTEOSTRACI. 



Scuti materia "lacunis" osseis et tubulis vascularibus numerosis instructa, superficie 

 tuberculis ornata. 



a. Oculis in medio scuto positis. 



Genus 1. Cephalaspis. — Scutum simplex semicirculare, cornubus lateralibus 

 instructum. 



Genus 2. Auchenaspis. — Scutum in duas partes, anteriorem et posteriorem divisum ; 

 anterior major, cornubus lateralibus instructa. 



Genus 3. Didymaspis. — Scutum in duas partes, anteriorem et posteriorem divisum; 

 anterior minor, sine cornubus lateralibus. 



