584 SUMMARY OF CURRENT RESEARCHES RELATING TO 
glands are also found ; the osphradia of the primitive Rhipidoglossa are 
found on the branchial nerve ; the sexes are generally separate, and in 
archaic forms of both groups there are no accessory glands or copulatory 
apparatus. 
In discussing the origin of the Lamellibranchiata it is as well to bear 
in mind that they cannot have given origin to the Rhipidoglossa, for 
the group of anisopleural Gastropods is, phylogenetically, more ancient 
than that of the Lamellibranchs ; this is shown by morphology, indi- 
vidual and phyletic development. 
Of all the Anisopleura the Rhipidoglossa are the most ancient ; this 
is shown not only by their bilateral symmetry and the freedom of the 
edge of the mantle, but by the absence of centralization of the nervous 
system, the embryonic character of the eye, in some of which the in- 
vagination-cavity remains open, and by the opening of the gonad into 
the right kidney. 
On the other hand, a number of characters show that the Lamelli- 
branchiata are more specialized than the Anisopleura in general, and the 
Rhipidoglossa in particular ; morphology, palaeontology, embryology, 
may all be summoned to testify to this. 
On the whole, Dr. Pelseneer concludes that the Lamellibranchiata 
are derived from forms of the rhipidoglossal type which have undergone 
no torsion, and he exhibits the relations thus : — 
Rhipidoglossa 
Lamellibranchiata 
Aplacophorus 
' Prorhipidoglossa 
Chiton /''' 
''v, r ' Archimollusc 
The Bulbils Arteriosus and Aortic Valves of Lamellibranchs.* — 
Prof. C. Grobben describes the post-ventricular bulbus arteriosus of 
Cytherea chione , Venus verrucosa , Mactra stultorum, and some other 
bivalves, and in part corroborates the recent investigations of Menegaux. 
The bulbus consists of interwoven smooth muscle-fibres extending 
through a matrix of connective tissue, and it contains a long valve 
which prevents regurgitation. Like Rankin and Menegaux, and others, 
Grobben finds a valvular arrangement at the beginning of the anterior 
aorta. In all the bivalves investigated there is a single semilunar 
valve, the nature of which in Pecten Jacobsens is described in detail. 
In the posterior aorta of Pecten Jacobseus, and probably in other Asi- 
phoniata, there is, besides the sphincter described by Dogiel, another 
valvular structure. 
* Arbeit. Zool. Inst. Univ. Wien (Claus), ix. (1891) pp. 163-78 (1 pi.). 
