LABIAL CARTILAGES OP RA1 A CLAVATA. 
109 
apparently wliat does take place in older specimens, for 
'Gunther (1870) says of this fish: “ The nasal valves confluent, 
without cirrus, forming together a simple broad flap in front 
of the month, the posterior edge of the flap being nearly free, 
not interrupted in the middle.” 
The nasal flaps of all of the Plagiostomi, whether Selachii 
-or Batoidei, are accordingly simply folds of the dermal tissues 
of the internasal portion of the snout, this internasal portion 
of the snout being presented more or less ventrally according 
to the greater or less development of the rostrum and the corre- 
lated configuration of the head. If the mouth were terminal 
and the nasal apertures disposed as in Amia and most of the 
Teleostei, this internasal region would lie on the dorsal surface 
of the snout, and the relations, anterior and posterior, would 
be the reverse of what they are in Raia. In the Batoidei the 
nasal flap always lies external to the nasal section of the 
latero-sensory canals, and the nasal-flap cartilage, which lies 
in large part in the flap, also always lies external to that 
canal, and external also to the nervus buccalis lateralis. In 
most Selachii the nasal flap lies wholly aboral to the nasal 
latero-sensory canal, that is, on the opposite side of the canal 
to the labial cartilages; but in my specimen of Scyllium it 
lies external to the canal, # as it does in the Batoidei. In 
Chlamydoselachus both labials lie oral to the suborbital 
latero-sensory canal but internal both to the third group of 
ampullae of Merritt Hawkes* (1906) descriptions and to those 
branches of the buccalis that supply those ampullae; the 
labials thus lying morphologically internal to the suborbital 
canal. In Mustelus (Allis, 1901) the labials have similar 
relations to the latero-sensory canals, ampullae, and related 
nerves. In my specimen of Scyllium the anterior end of the 
single upper labial (Gegenbaur, 1872) lies directly internal to 
the latero-sensory canals at the point where the nasal canal 
joins the suborbital canal ; and, in Stegostoma tigrinum, 
Luther (1909) says that the rostral (external) surface of the 
-anterior labial is grooved to lodge the nasal canal. 
The so-called anterior upper labial of Gegenbaur's descrip- 
