110 



PROF. P. M. DUNCAN ON THE 



regularly placed, elongate or circular, low projections become 

 visible on the broad inferior portion of the plate ; and on the 

 upper part four vertical double rows of foramina. The double 

 rows are separated by low horizontal ridges, and a median low 

 ridge is placed vertically along the median line so as to separate 

 tlie foramina into two lateral series. The foramina are large and 

 pass quite through the jaw-plate (figs. 2 & 3). 



The slightly expanded bases of the tooth -papillae (fig. 4) cover 

 the low projections, but the base of a tooth covers two of the 

 horizontally placed foramina besides some of the surface of the 

 ridges just above and below them (figs. 2, 3, 6). The outer, or 

 rather the side, tooth-papillae are usually with elongate bases, and 

 are placed on correspondingly shaped projections which are more 

 or less oblique, and the papillae which are along the median line 

 of the plate are upon wider apart and circular low knobs. 

 The papillae (fig. 4) have their bases hollowed out so as to 

 fit the projections, and connective tissue unites them with the 

 plate around the edges of the projections. The highest papillae 

 are two in number, and resemble a true tooth divided along the 

 median line and with an incurved free edge (fig. 5). But there 

 are really two papillae, and their bases are wide and elliptical in 

 shape ; moreover each has a pair of depressions in the base, and 

 the union with the jaw-plate is by connective tissue only along a 

 wide space (fig. 2). No muscular structures are found connected 

 with the tooth-papillae. 



The four teeth are long, broad and thick at the base. They 

 are composed of opaque carbonate of lime except at their free, 

 sharp, yet broad, edge, where the mineral is semitransparent 

 (fig. 6). On separating the teeth from the jaw-plate, it will be 

 observed that their bases are elliptical and broad as well as 

 hollowed-out. There is a double hollowing, and each hollow 

 corresponds with a foramen in the jaw-plate (fig. 2). The upper 

 and lower edges of the bases rest on the ridges between the pairs 

 of foramina. 



A small muscle passes out of each foramen and is inserted at 

 the hollow of the base of a tooth, so that a tooth has two muscles, 

 one on each side of the median line, and the direction of the 

 muscles is rather oblique (figs. 3 (c), 7). 



It may b(; noticed in some specimens that the side rows of 

 tooth-papilla arc close to their neighbours of the next plates, and 

 that on turning the animal on its hack a very decided funnel- 



