THE ELASMOBRANCH FISHES 97 



region of the skull to the tip of the tail. These fillers universally take a horizon- 

 tal direction. Those fibers between the lateral line septum and the midventral 

 line are divided into a lateral (l.h.) and a ventral (v.d.) region. The lateral 

 muscle (l.h.) is readily recognized by the fact that it lies just ventral to the 

 lateral line and is of a dark color. Between pelvic and pectoral regions this 

 bundle is folded in in such a way (figs. 99 and 100) as to be overlapped by the 

 ventral bundle (v.h.). Anterior to the pelvis the fibers in the lateral bundle 

 may take an almost horizontal direction (l.h., fig. 112) but they usually take 

 an oblique direction of anterior and 



upward (fig. 100) ; while posterior to .^''-'r'^''^f^i^'. 



the pelvic area the fibers of this bundle 

 run more or less horizontally. 



The ventral bundle {v.h.,^gs. 100 and 

 112) is highly specialized and its fibers 

 take a characteristic anteroventral di- 

 rection. In some of the Elasmobranchs, 

 as in Zygaena, the ventral bundle is 

 further specialized into a rectus ab- 

 dominis at the midventral line (r.a., 

 fig. 100). 



Figure 99 is a model for Squalns 

 siickUi of all muscle fibers between two 



myosepta in the region of the first dor- Tig. 98. Showing the finer anatomy of de- 

 , „ T^ m ., Ml 1 ,T , ,1 veloping muscle. (Modified from Erik 



sal nn. Dorsally it will be seen that the Miiiier.) 



septa run posteriorly close together 



and almost parallel with the middorsal line. They then turn anteriorly and 

 run forward. Next they bend on an acute angle and curve backward; and then 

 they turn forw^ard and downward to the lateral line, where they fold inward 

 and are carried forward. When the septa emerge from the fold they are dis- 

 placed backward so that as they pass through the lateral bundle they are fully 

 a half -segment posterior to the same myosepta above the lateral line. In the 

 lateral bundles the septa curve downward and backward and then turn for- 

 ward where they fold inward and backward to emerge in the ventral bundle 

 (v.h.). In the ventral bundle they run sharply downward and slightly forward. 

 They then turn backward, and finally run forward to the midventral line. 



In a transverse section of the tail of Lamna the various muscle cones appear 

 in their relation to one another (fig. 101). The rows here are so arranged that 

 a dorsomedian {dm.), a dorsolateral {dl.), and a lateral row {l.h.) lie above 

 the lateral septum which extends from the spinal column to the skin in the 

 region of the lateral line (not drawn in the figure). The ventrolateral {vl.) 

 and ventromedian {vm.) rows are below the septum. The bundles are made up 

 of concentric lamellae, the concentricity of which is due to the projection of 

 the apex of one cone into the angle of the V's in front of or back of it. The 

 dorsolateral and ventrolateral bundles are small, the dorsomedian and ventro- 

 median bundles are large, and the lateral bundle, composed of nine concentric 



