420 OEGAIS- OF JACOBSOX IN SPHEKODOK. 



EXPLANATION OP THE PLATES. 



C.C.U., L'artibigu of the external nares ; c.n., external uares ; J.O., Jacobson's 

 organ: l.d., hiciirjiual duct; l.J.c., lower part of Jaeobson's cartilage ; Mx. 

 maxilla ; A"((., nasal ; n.c, nasal cartilage ; n./.c, nasal-lloor cartilage 

 U.S., nasal septum ; Pmx., i^remaxilla ; ^jt.s.c, paraseptal cartilage ; F.Vo., pre- 

 Tomer ; tS.JLv., septo-maxilla ; tc.J.c, upper part of Jaeobson's cartilage. 



Plate 41. 

 Fig. 1. Transverse sectioi^ of nose of embryo Spliciwdon jyioivtatus, SI age E, 

 showing the broad expansion of the lower part of the nasal septum 

 immediately behind the premaxillaries. The section is through the 

 anterior nares, and a portion of the supporting cartilage is seen 

 above and below. 



2. Transverse section a short distance behind fig. 1. The lower part of 



the septal cartilage sliows four excavations. The upper two are for 

 Jaeobson's organ, the lower for the prevomers. The section shows 

 the anterior part of the septo-maxiUarj. 



3. Transverse section a little behind fig. 2. The anterior part of Jacob- 



son's organ is seen, also the prevomers. The lower part of the 

 septal cartilage is now seen forming a papillary cartilage {p.c). 

 The septo-ni axillary is of large size, and forms part of the external 

 wall of the skull. 

 Fig. -1, fig. 5, and fig. G are almost consecutive sections a little behind fig. S. 

 Fig. 4 shows a section of the organ where it is opening into the 

 anterior and inferior corner of the nasal cavity in common with the 

 lachrymal duct. The organ is roofed above and below by cartilage. 



Plate 42. 



Fig. shows the organ just behind its opening into the anterior part of the 

 nasal cavity ; the cartilage of the o)-gan shows an upper and a lower 

 part, the lower being manifestly the parasejDtal cartilage. The lower 

 part of the nasal cartilage is separated as a nasal-fioor cartilage, 

 almost exactly as is seen in most mammals. Fig. 6 is a section which 

 in almost every respect is typically mammalian. "With the exception 

 of the fact that the mode of opening of the lachrymal duct is 

 different, the section might pass for a section through the snout of a 

 marsupial. In the marsupial, as in most mammals, Jaeobson's 

 cartilage is supported by the palatine process of the premaxilla : here 

 the supporting bone is the prevomer. 



Fig. 7. Transverse section a considerable distance behind fig. G. The para- 

 septal cartilage is small and the prevomer large. The lachrymal 

 duct is seen below the maxilla and the pi-efrontal. 

 8. Transverse section of snout of Dasi/ptis villosu.^. Below the base of 

 the true nasal septum are seen the anterior portions of the cartilages 

 which become Jaeobson's cartilages. Along the inner part of the 

 nasal Hoor is seen the well-developed bone wliich is believed to be 

 the septo-maxillary (S.Ma:). A little farther back it lies inside of 

 the premaxillary, and has no connection with uny cither bone. 



