26 THE AUSTRALIAN MUSEUM. 



Mantle yellow, opaque, muscular. The muscle bundles are strong, 

 but loosely arranged. The chief ones run radially from the siphons and 

 circularly round the body. 



Branchial sac with 8 well-marked folds on each side, up to 12 internal 

 longitudinal bars on a fold, and 3 or 4 in the interspace. Occasional 

 transverse vessels very much wider than the rest. Meshes rather 

 small, elongated transversely, containing about 6 stigmata ; horizontal 

 membranes well-marked. 



Dorsal lamina a plain smooth-edged membrane. 



Tentacles about 18, 9 large and much branched, and about the same 

 number of much smaller intermediate ones. 



Dorsal tubercle prominent, with both horns coiled inwards, sometimes 

 so much coiled as to form a pair of conical spirals. 



Locality. Port Jackson ; about 20 specimens. 



This species probably a common one at Port Jackson has the 

 characteristic external appearance of a Microcosmus. The actual shape 

 differs much in the various specimens (see PI. Cyn. VI., figs. 1, 2, 3). 

 Several of the smaller specimens have the apertures on prominent 

 siphons, of which the branchial may be very long (fig. 3) ; the colour in 

 these young specimens is whiter. In older specimens the siphons are 

 less marked, and the shape is more irregular. The size varies from 

 5 cm. down to about 2 cm. in length. There are four that measure 

 about 4 cm. by 3 cm. by 2 cm. Then there are the four shown in fig. 2, 

 some others more irregularly shaped, and half-a-dozen small ones, 

 like fig. 3. The invaginated test lining the branchial siphon is armed 

 with numerous little spines closely placed. Near the base of that siphon 

 there are four pouch-like folds forming a partial diaphragm (fig. 5) ; and 

 in the atrial siphon there is a single large crescentic fold springing from 

 the anterior side, and forming a valve or partial septum. 



The loosely arranged muscle bundles of the mantle are in places 

 closely attached to the inner surface of the test. 



In the branchial sac there is a very slight 9th fold on each side, next 

 to the endostyle, and only extending for part of the length of the sac. 

 Figure 4 shows one of the very wide transverse vessels. The dorsal tubercle 

 (fig. 5) is in some specimens a good deal more coiled, so as to form a 

 pair of prominent cones. 



This species of Microcosmus and specimens of Styela personata 

 apparently occur together, as they were mixed in several packages. 

 Externally they are very much alike, and it is impossible to tell the 

 Microcosmus from the Styela until the branchial sac or tentacles have 

 been examined. 



