IIALOSAURUS ATTENUATUS. 297 



cavidal region. Greatest depth one sixteenth of the entire length. Head 

 elongate, one eighth of the total length, retaining a considerable width for- 

 ward but losing in depth. Snout more than half as wide as the head, preoral 

 portion equal to half the length from the eye, broad, rounded and shovel- 

 shaped at the end. Rostral cartilage with three longitudinal ridges below, 

 bearing a series of prominences across the under side of the middle, blunt 

 angled and rather wide at the end. Mouth medium, about twice as wide as 

 long; maxillary extending little below the eye, with a sharp spine on its 

 upper angle at the end. Teeth small, in villiform bands, similar to those of 

 H. radiatiis but more slender, on jaws, palatines, and pterygoids. Eye 

 medium, length more than twice the width of the interorbital space, more 

 than five and one half times in the length of the head, two and one half 

 times in the length of the snout. Nostrils small, close together, close to the 

 orbit, anterior with a hood-like valve opening forward. Opercles thin, 

 flexible, rather short, the muciparous canals extending farther back and end- 

 ing in a couple of angles below the base of the pectoral. The opercle itself 

 is dark colored and, apparently, the whitish membranes of the canals are 

 applied to its surface. Gill arches rather short ; eleven rakers on the first 

 arch, shorter than the laminae, tubercular. Gill membranes hardly united, 

 free from the narrow isthmus : gill laminte short, two fifths as long as the 

 eye. Mucous canals greatl}' developed along the side of the head and below 

 each lower jaw ; that from the snout below the eye to the opercle is met by 

 that from the chin below the pupil, both widening as they pass backward 

 until at the end their width equals the length of the orbit. On the top of 

 the head the development of the mucous sj'stem is hardly greater than on 

 the flank. From the upper angle of the gill opening the lateral line system 

 drops into and through the axil, below the base of the pectoral, until low 

 on the flank where it passes backward, traced by an oi)aque whitish band 

 (the nerve) under a series of scales upon wdiich there is a series of vertical 

 organs, probably light producers, which externally are covered by a thin 

 transparent membrane. The vertical organs resemble those of Lampro- 

 grammus, Plate XXXIV. fig. 5. 



Dorsal origin little more than two lengths of the head from the snout ; 

 base twice as long as the e^-e, the same distance backward of the insertions 

 of the ventrals ; fin shaped like that of H. macrocldr, higher than long, rays 

 decreasing rapidly in length from the second backward; first ray shorter 

 than the second, slender. Origin of the anal about three lengths of the 



