m 
The Skeleton. 
Figure 32 represents the lateral aspect of the chief skeletal parts 
of the head region. As in Geotria, the parts most worthy of notice 
are the piston (P.C.) and median ventral (M.V.) cartilages. The latter 
(figs. 33, 34) is a narrow transverse bar, pointed at each end and 
bearing a median posterior projection. The former (figs. 33, 34) is 
similar in general form to that of Petromyzon but differs somewhat 
anteriorly. The anterior tooth-bearing “head” of the piston cartilage 
is a massive piece of cartilage bearing on its anterior ventral aspect 
the two pairs of rasping cusp-bearing plates or “teeth”. The anterior 
pair of piston teeth are triangular plates, their length directed longi- 
tudinally and bearing numerous small cusps; the hind pair are much 
smaller than the auterior, are of somewhat similar shape and fused 
together in the middle line. 
Appendix. 
I have thought it well to add to the figures of dissections of Geotria 
some semi-diagrammatic figures (35—37) of transverse sections taken 
through the head of the Velasia stage, since these latter explain more 
clearly than any words the inter-relationships of muscles and cartilages. 
I have introduced references in the text to these transverse sections. 
Figure 35 represents semi-diagrammatically a thick transverse 
section through the Velasia head at the level of the anterior end of 
the piston cartilage (P.C.). This and other cartilages (calcified and 
uncalcified) in figures 35—37 are coloured black. The space labelled 
“throat cavity’” in this figure should perhaps more properly be termed 
the hind part of the funnel cavity. The sections being thick, the position 
of the attachments of the hyomandibulari-glosses muscles (X XJ) and 
forward-extending inner ventral bands of the basilaris (X_XJ) are shown. 
Comparison of this figure 35 with figures 20 and 21 should suffice 
to explain the appearance of the various muscles and other structures 
shown in this transverse section (about 2 mm. thick). 
Figure 36 represents semi-diagrammatically a more posterior section 
taken at the level of the most anterior segment of the dorsal myotomes 
(Ill). Comparison with figures 18—-22 and 27—29 will suffice to 
explain its appearance. The relationships of the pharyngeus (XX) 
musculature are clearly shown. 
Figure 37 represents semi-diagrammatically a transverse section 
