1927] Pilsbry-Bequaert, The Aquatic Mollusks of the Belgian Congo 299 
gastropods, or referred to Mesozoic genera of marine or fresh-water 
shells. KE. A. Smith has referred part of the genera to the Melaniide 
(=Thiaride), leaving many others without family affiliations. Pel- 
seneer’ has placed Spekia, Tanganyicia, Limnotrochus, and Chytra in the 
family Hydrobiide (=Amnicolide), Nassopsis (=Lavigeria) and By- 
thoceras in the Melaniide, and Tiphobia and Bathanalia in the Tipho- 
biide. In a note written later? he concluded “that all the ‘halolimnic’ 
_ forms belong to the family Melaniide or to very closely related types.” 
We agree with this conclusion. With the exception of the Syrnolopsi- 
dee, the entire series, so far as their structure is known, appears to be 
melanid, Certainly none of them are hydrobiid and the characters 
assigned for the Tiphobiide do not distinguish that group from Melaniide. 
Thiele’s view of Lavigeria was alluded to on p. 249. 
\ 
Fig. 56. Opercula of: a, Limnotrochus thomsoni E. A. Smith; b, Paramelania 
damont (EK. A Smith); c, Lavigerta grandis (K. A. Smith) (copy of Smith’s figure). 
The opercula of these shells are of three types, as follows: © 
1.—PavcisPIRAL (Fig. 56a), with nucleus below the center or at 
about the lower third: Bathanalia, Limnotrochus, Chytra, and Giraudia. 
This is the well-known littorinoid or ‘‘paleomelanian’”’ operculum, a 
primitive type found in many diverse thiarids. 
i 2.—SuUBSPIRAL (Fig. 56c). These genera have the extreme form of 
the ‘“‘neomelanian” type, with marginal nucleus: Lavigeria and EKdgaria. 
This type of operculum is found also in many branches of the family of 
melanians, doubtless evolved from the paucispiral type. 
38.—CONCENTRIC AROUND A PavcisprraAL CENTER (Fig. 560): ° 
Tanganyicia, Reymondia, Tiphobia, Paramelania, Bythoceras, and 
Spekia. This relatively evolved form of operculum occurs also in Cleo- 
patra and Paludomus among the melanians. Also in Bulimus (= Bithy- 
nia), Lioplax, and elsewhere. It has evidently arisen independently 
many times from the paucispiral type. 
The radulz are of two types: 
11906 ‘A Treatise on Zoology, V, Mollusea,’ Dp. 154. 
21907, Rept. Brit. Asso. Adv. Sci., (1906), p.602. 
