CHAPTER TWO 



Chimaeroids' 



BY 

 HENRY B. BIGELOW and WILLIAM C. SCHROEDER 



GENERAL DISCUSSION 



The scope of this chapter, the sources of material, the method of measuring, and 

 the bases for the discussions of habits and geographical distribution have been the 

 same generally as for the chapter on the batoid fishes (p. i). The illustrations are by 

 E. N. Fischer. 



Subclass HOLOCEPHALI 



Characters. Cartilaginous fishes (Chondrichthyes) with a single external gill opening 

 on either side covered over by an opercular fold of skin with cartilaginous supports and 

 leading to a common branchial chamber into which the true gill clefts open; five gill 

 arches, four pairs of gill clefts, and four pairs of gills; upper jaw (palatoquadrate cartilage) 

 fused throughout its length to lower surface of cranium, without visible trace of demark- 

 ation between the two; the two branches of lower jaw (Meckel's cartilages) fused to- 

 gether in front and suspended behind only from a prominence from rear end of upper 

 jaw cartilage; no connection between lower jaw and hyoid arch; no distinct hyomandib- 

 ular cartilage,^ though it is perhaps represented by a small cartilage connected above 

 with the hyoid; three rostral cartilages articulated to front of cranium; cranium with a 

 horizontal transverse septum dorsal to anterior part of brain case; auditory capsule with 

 incomplete inner (median) wall; no distinct vertebral centra; notochord not constricted 

 segmentally but surrounded in most by numerous calcified rings; the two halves of the 

 pelvic girdle united below by a ligament only; teeth represented by two pairs of dental 



1. Contribution No. 571 from the Woods Hole Oceanographic Institution. 



2. There is such a hyomandibular cartilage with which the lower jaw is connected in all elasmobranchs except for the 

 notidanoid sharks (Notidanoidea). 



