PLATE I. 
Spiruin ptcronii, female (Challenger specimen). 
Fig. 1. — Left side. 
Fig. 2. — Ventral side. 
Fig. 3. — ])orsal side. 
Fig. 4. — Posterior or aboral end. 
Fig. 5. — Dors;il side ; the free mantle is cut along the median dorsal line and the 
halves divaricated. 
Fig. G. — Ventral side, the free mantle laid open in the median line and- the lateral 
portion divaricated; moreover, the right wall of the “episoma” is 
removed so as to lay open the right half of the viscero-pericardial cavity. 
In all the figures : 
a, h. remains left b}' the colouring matter of the irides of the eyes on the 
inner surface of the mantle in consequence apparently of the strong 
contraction upon them, 
on. Anus. 
hr. 
cU. 
K. FJ. 
F. F'. 
fi- 
le. 
ik. 
Ir. 
U. W. 
hfj. r,</. 
ol. oV. 
o.m. 
o.fl. 
Gills. 
Median dorsal infundibular apophyse (nuchal cartilage). 
Eyes. 
Fins. 
Terminal fossa and papilla. 
Collar of the infundibulum. 
Ink ba". 
O 
Infundibular valve. 
Plight and left infundibular apophyses. 
Nidamental glands. 
Olfactory organs. 
Miuss of the ovar)'. 
Oviduct. 
])h. pi/. Palliobranchial ligaments. 
/". Mantle. 
F.fl. P.l. F.l'. I)orsal and lateral lobes of the free edges of the mantle. 
Fk. Shell seen through the ventral and dorsal openings. 
I. T . Tentacular anns. 
Acce.ssory nidamental glands. 
I, II, III, IV, V, IP, IIP. IV', brachia. 
