N. ROSÉN, STUDIES ON THE PLECTOGNATHS. 17 



mens of Spheroides tesiudineus, as well as on studies of mic- 

 roscopical sections of young specimens (18 mm. in length) 

 of the same species. 



In a young specimen measuring 18 mm. in length the 

 cJiondrocranium bas the following structure (Fig. D). There 

 is an unpaired narrow trabecle, which ends anteriorly with 

 two very small divergent horns. It forms a narrow ethmoid 

 plate. The anterior part of the trabecle is connected with 

 the anterior cranial roof (tectum cranii ant.) by a nasal sep- 

 tum. This continues caudally between the anterior parts of 

 the orbits. Along its caudal margin the membranous inter- 

 orbital septum inserts. There is a small planum antorbitale 

 on each side. The olfactory nerves run on each side of the 

 nasal septum and penetrate plan. orb. From the caudal 

 margin of the anterior cranial roof two narrow cartilaginous 

 chords take their origin. They unites caudally to form a 

 posterior cranial roof of a triangulär shape. A diagram of 

 the condrocranium is given in Fig. D. This shows that the 

 cranial roof is very incomplete. It is divided by a large 

 fontanelle into two portions, connected by a pair of chords. 

 The posterior cranial roof tapers caudally to a point (taenia 

 med.). The septal cartilage is not excavated by the so-called 

 anterior canal of the eye-muscles, as is usiially the case. The 

 cranial roof is connected with the cerebral chondrocranium 

 by short taeniae marginales. The tectum synoticum is rather 

 well developed. The otic capsules form in the specimen in 

 question mere slight prominences of the cranial capsule. I 

 have found only two cartilaginous semicircular septs, which 

 represent the lateral and the posterior ones. The trabecle 

 ends posteriorly blindly, not being in connection with the 

 basis cranii. This is well developed. There is only one fe- 

 nestra basis cranii, which is long and narrow. It is border- 

 ed laterally by two processes of the basilar plate. The 

 anterior end of the chorda is enclosed perfectly in the ba- 

 silar plate. The so-called posterior canal for the eye-muscles 

 (m. rect. lat.) has a membranous roof throughout its whole 

 length. The lateral rectus muscles pass through the fenestra 

 basis cranii and insert to the upper surface of the para- 

 sphenoid, but do not continue caudall}^ between this bone and 

 the basilar plate, as in the salmon, in which species the roof 

 of the posterior part of the canal mentioned is cartilaginous, 



Arkiv för zoologi. Band 10. X:o 8. 2 



