176 IOWA GEOLOGICAL SURVEY 



Mass. Although incomplete, the part that is preserved shows 

 several of the cranial elements very distinctly, and also the 

 narrowed posterior portion of the parasphenoid which sup- 

 ports the notochordal sheath. This is deeply channeled in 

 the median line for the passage of the notochordal sheath, and 

 is concentrically striated in the same manner as the vertical 

 lamina which descends from the posterior margin. The sheath 

 itself exhibits no trace of segmentation, and, like that in the 

 type species, is of remarkably small diameter. 



Formation and locality. Middle Devonian; Eifel district, 

 Rhenish Prussia. 



Macropetalichthys pelmensis Hennig. 



1907. Macropetalichthys pelmensis E. Hennig, Centralbl. fur Min. Geol. Pal. 

 no. 19, p. 589, text-figs. 1-3. 



Under the above name has been recently described a new 

 species of Macropetalichthys which agrees very closely in form, 

 size and general appearance with the type of M. agassizi (von 

 Meyer), and indeed is only distinguishable from it by means 

 of its finer ornamentation and gently ridged condition of the 

 headshield over the occipital region. The cross-section of the 

 latter shows that the cranial roof rises into a low peak in the 

 median line, and slopes away rather abruptly laterad of the 

 occipital sensory canals. The latter are conterminous with the 

 posterior margin of the headshield, at which point they are de- 

 flected abruptly downward and also at a slight angle inward, be- 

 ing encased in a funnel-shaped duct which is partially filled with 

 cancellated tissue. Identical conditions have been observed in 

 the type species of Macropetalichthys, and also in M. agassizii. 

 The peculiar structures in question are in nowise homologous 

 with the articular sockets for antero-dorso-lateral plates in other 

 Arthrodires, as supposed by Hennig, but are clearly the continua- 

 tion of the sensory canals from the cranial roof downward into 

 the interior of the headshield, where they were probably in 

 communication with the internal auditory organs. According 

 to this interpretation the funnel-shaped orifice would corre- 

 spond to the ductus endolymphaticus, which in modern Elas- 

 mobranchs opens on the dorsal surface of the head by an 



