The Ayiatomy of Chlamydoselachus 



389 



muscles of the body and tail — as in Scyllium (Van Wijhe, 1883, p. 36 and Fig. 25, Taf. 

 Ill); in Lacerta (Corning, 1895); in Fctromyzon and Squalus (Neal, 1897); and in Lepido- 

 siren and Frotopterus (Agar, 1907). 



In Heptanchus (Davidson, 1918) the following 

 muscles (Text-figure 63) are recognized as members 

 of the hypobranchial group : the paired coracoarcu- 

 ales communes (car.), the unpaired coracomandib- 

 ularis (c.md.), the paired coracohyoidei (c.hy.), and 

 seven pairs of coracobranchiales (c.br.1-7). In elas' 

 mobranchs generally, according to Daniel (1934, p. 

 108), all of these muscles excepting the coraco' 

 branchiales arise from the first five trunk myotomes. 

 Edgev^orth (1903) states that in Scyllium the coraco- 

 branchiales develop from head myotomes. In the 

 adult Heptanchus, the metameric nature of the cora' 

 coarcuales is attested by the presence of a series of 

 four transverse or sHghtly oblique myosepta (Text- 

 figure 63). In the coracoarcuales of Scymnus, there 

 are five such myosepta (Fiirbringer, 1897, Fig. 3, 

 Taf. VI). In Heptanchus, Vetter (1874, Fig. 9, pi. 

 XV) shows a myoseptum in the coracohyoideus 

 muscle also. 



The hypobranchial group of muscles is often 

 called the hypoglossal musculature because the mus- 

 cles of this group are supplied, somewhat indirectly, 

 by a nerve which, variously called the spino-occipi- 

 tal, occipital or hypoglossal nerve in fishes and am- 

 phibians, in the higher vertebrates is known as the 

 hypoglossal (hypoglossus) or twelfth cranial nerve. 

 This nerve is a composite structure, made up from 

 a series of roots representing, perhaps, several 

 neuromeres. 



AlHs (1917 and 1923) does not distinguish the 

 hypobranchial muscles of Chlamydoselachus as a 

 separate group. However, he describes the distribution of the branches of "a large 

 nerve which was not traced upward to its origin, but which is either of spinal, or spinal 

 and occipital origin" (AUis, 1923, p. 195). The muscles supplied by this nerve are 

 identical with those included in Davidson's list of hypobranchial muscles in Heptan- 

 chus, with the addition of a muscle which AlHs calls the "pharyngo-clavicularis." The 

 hypobranchial muscles of Chlamydoselachus are shown, in color, by AlHs (1923) in his 

 Figs. 35 and 37-40, pis. XIII-XV; but nowhere are these muscles of Chlamydoselachus 



Text-figure 63. 



Hypobranchial muscles of the notidanid, 



Heptanchus maculatus, ventral view. 



bh., basihyoid cartilage; car., musculus coracoar- 

 cuales; ch.l, first ceratobranchial cartilage; c.br.1-7, 

 first to seventh coracobranchial muscles; ch., 

 ceratohyoid cartilage; c.hy., musculus coracohyoi- 

 deus; CO., coracoid cartilage; c.md., musculus cora- 

 comandibularis; ibu.HS, first to sixth ventral 

 interbranchial muscles; md., mandibular cartilage. 

 After Davidson, 1918, Fig. 4. 



