MUSEUM OF COMPARATIVE ZOOLOGY. 85 



angular, but does not cross the symphysis. The orbito-nasal is long. In pass- 

 ing the nostril, the short subrostral makes a decided curve. Behind the nostril 

 the nasal is bent toward the mouth. The median is very short. From the 

 median the short prenasal goes directly to the side, joining the rostral. 



By the canals either Odontaspis or Ginglymostoma shows more affinities with 

 Heterodontus than does Acanthias. The differences in dentition between the 

 latter genera are scarcely greater than those apparent in the canal systems. 



Acanthias. 



Acanthias americanus (Plate XIX.). Backward from the shallow scapular 

 curve, the laterals of this species are nearly straight. Above the widest part of 

 the lower lobe of the tail, the tube makes a slight bend upward ; it does not 

 follow the vertebral column, but gradually approaches the lower edge of the 

 muscles, and stops in front of the last vertebra. 



In the middle, towards the ear openings, the aural is bent forward. A con- 

 tinuous longitudinal line is formed by the elongate occipital and the lateral. 

 At the orbitals, the cranials make a rather sharp curve; opposite the fonta- 

 nelle, they make a broad and shallow bend. The upper portion of the orbital 

 is sinuous; behind the eye it is thrown backward; and beneath the orbit it 

 goes but half-way before turning back in a sharp angle to join the angular. 

 Jugular and angular together are short. The orbito-nasal is long, and is bent 

 downward from the suborbital. By the side of the nostril there is a decided 

 bend in the subrostral. The nasals are long and bent so that the curves in 

 each approach the outlines of a Z; they do not meet to form a median, but 

 run close together as in Pristiophorus. Near the end of the snout the pre- 

 nasals converge, without seeming to join; they are located some distance from 

 the rostrals. The tubes are of large calibre, and the tubules are numerous 

 and short. A short oral lies close to each angle of the mouth, entirely dis- 

 connected. On the tail, for a short distance from the end, the canal is open. 

 Figure 6 shows the arrangement of the scales and the form of the portion of 

 the canal included between the dotted lines. 



An embryo of two and a cpuarter inches has tubes similar to those of the 

 adult, but the tubules are shorter or absent. 



Somniosus- 



Somniosus carcharias (Plate XX.) has tolerably straight tubular laterals. 

 They extend on the middle of the muscular portion of the tail, running as far 

 back as the hinder edge of the anterior lower lobe of the caudal fin; thence 

 they descend to the lower edge of the muscles, above the fibrous portion, where 

 they continue to the end of the column. 



Among the cephalic canals a very peculiar arrangement occurs on the occi- 

 put: the aural is transverse, and has its ordinary position; from its ends the 

 occipitals curve forward and inward, and end anteriorly without connecting with 



