M or dacia—STB. AH AN 
419 
TABLE 2 
POSITION OF THE Cloaca in Ammocoetes OF Mordacia mordax AND Geotria au sir a is 
M. mordax 
G. australis 
Total 
Head to cloaca 
No. of 
Total 
Head to cloaca 
No. of 
length 
precloacal 
length 
precloacal 
(mm.) 
mm. 
% of T. L. 
myomeres 
(mm. ) 
mm. 
% of T. L. 
myomeres 
102 
84 
82 
104 
37 
29 
78 
92 
111 
93 
84 
103 
44 
35 
80 
90 
121 
101 
83 
102 
52 
41 
79 
91 
130 
109 
84 
103 
| 55 
44 
80 
90 
131 
111 
85 
101 
61 
50 
82 
93 
132 
110 
83 
101 
73 
57 
78 
89 
135 
113 
84 
103 
83 
64 
77 
91 
140 
118 
84 
103 
84 
64 
76 
95 
145 
120 
83 
102 
87 
67 
77 
90 
160 
135 
84 
103 
89 
68 
76 
92 
LI 
00 
103 6 1.0 
78 ± 2% 
91 ± 1.7 
It is noteworthy that the largest ammocoete 
of M: mordax in the sample (160 mm. long) is 
much longer than the largest ammocoete of 
Geotria recorded by Maskell (1929 ) from New 
Zealand (104 mm. long), or by the author, 
from Western Australia (100 mm. long). This 
is the opposite condition to that of the adults. 
Fourteen specimens of M . mordax in the Na- 
tional Museum of Victoria, Melbourne, ranged 
from 21 to 42 cm. in length (average 33 zb 5 
cm.), while 22 specimens of G. australis from 
the same collection ranged from 34 to 58 cm. in 
length ( average 47 zb 7 cm. ) . This condition 
of a relatively larger larva and relatively smaller 
adult suggests that, compared with Geotria, M. 
mordax has a tendency to neoteny. 
SUMMARY 
A single macrophthalmia stage of Mordacia 
mordax is described. Its dentition is very similar 
to that of the adult and thus is different from 
that of Geotria australis. 
Although in general body form the ammo- 
coetes and the macrophthalmia stages of the 
two species are very similar, the cloaca is more 
posterior in Mordacia , lying at 84 zb 1 per cent 
of the total length of the body, well behind the 
origin of the second dorsal fin, and with 103 zb 
1 precloacal myomeres. Geotria has the cloaca 
at 78 zb 2 per cent of the total length, just below 
the origin of the second dorsal fin, and with 
91 zb 1.7 precloacal myomeres. This character 
serves to distinguish premetamorphic stages of 
the two Australian species. It is also relatively 
constant in older stages, except in the secondarily 
shortened mature adults of Geotria . 
The ammocoetes of M . mordax grow to a 
greater length than those of G. australis, whereas 
the adults are shorter. This suggests a relative 
tendency towards neoteny in M. mordax. 
ACKNOWLEDGMENTS 
The author wishes to express his thanks to 
the curators of the Western Australian Mu- 
seum, the National Museum of Victoria, and the 
British Museum (Natural History), for grant- 
ing access to their collections of lampreys. 
REFERENCES 
Holly, M. 1933. "Cyclostomata.” In: F. E. 
Schulze, W. Kiikenthal, K. Heider, and R. 
Hess, Das Tierreich, Lfg. 59, de Gruyter, 
Berlin and Leipzig, xiv + 62 pp., 57 figs. 
Lahxlle, F. 1915. Apuntes sobre las lampreas 
Argentinas. An. Mus. Nac. Hist. Nat. B. Aires 
26: 361-382. 
Mann, F. G. 1954. La vida de los peces en 
aguas Chilenos. Universidad de Chile, San- 
tiago de Chile. 342 pp., illus. 
