490 



Bashford Dean Memorial Volume 



Text-figure 125. 

 Innervation of the senson,' canal system and certain of the pit organs in Squalus acanthias. 



hu.Vn, buccalis nerve; cc, supratemporal canal; dr.X, ramus dorsalis of tenth nerve; hmc., hyomandibular 



canal; ioc., infraorbital canal; 1!., lateral Kne canal; IIX, lateral line nerve; mc., mandibular canal; mde.VlI, 



external mandibular nerve; os. VII, ophthalmicus superficialis of seventh nerve; po., pit organs; soc., supraorbital 



canal; st.IX, supratemporaHs of ninth nerve; st.X, supratemporalis of tenth nerve. 



From Daniel, 1934, Fig. 245; after Nonis and Hughes, 1920, Fig. 50. 



Hawkes, the oral, gular and spiracular are labeled HLA, HLB and HLC respectively. 

 The preceding statements concerning the open condition of the canals hold for my four 

 large specimens, save that on the right side of No. I the groove is lacking for a distance 

 of about 30 mm. from the tip of the tail. 



A more extensive occurrence of sensory canals as open grooves is found in the 

 Holocephali, where most of the canals, including those of the head, are open; but in the 

 Selachii, Chlamydoselachus appears to be unique in the extent to which its sensory canals 

 are open. The nearest approach to its condition in this respect is found in the notidanids 

 (Daniel, 1934j, where the lateral line is an open groove as far forvv^rd as the pectoral fin. 

 In Heptanchus the canals of the head are all closed tubes, as far back as the fifth gill-cleft. 

 Posterior to this, the lateral lines are represented by a pair of open grooves extending 

 almost to the tip ot the tail. In Squalus (Text-figure 125) the canals are closed excepting 

 in the region tov."ard the tip of the tail. In higher elasmobranchs, the canals are usually 

 closed throughout their entire length. 



The open condition of the sensory canals found by Garman in Chlamydoselachus 

 (Text-figure 126) is probably primitive, and in the light of all the evidence can scarcely 

 be explained as due to arrested development in the embryonic sense. Lateral Hne canals 

 as open grooves were found by Dean ( 1909, p. 252; m the Devonian fossil shark Ctenacaiv 

 thus clar\^i (Text-figure 127,) as well as m many acanthodians. In all these forms the 

 dermal denticles terminate abruptly at the margins of the groove, and the marginal 

 denticles are, in most instances, unusually large, precisely as they are in Chlamydoselachus. 



Most of the terms used by Garman in describing the sensory canals of the head in 

 his specimen have been abandoned, and in their places are names for the \^arious divisions 



