6 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 20 



ten different genera, including Septifcr Recluz, Arcoperna Conrad, and 

 Adula Adams. Idasola is included in or made a synonym of Adula. 

 Lamy's work is very valuable, as he has made a thorough study of the 

 synonymy of most of the species and also mentioned species not preserved 

 in the Paris museum. Unfortunately he did not give diagnoses of the 

 various species and the soft parts were not taken into consideration. Some 

 of his conclusions may be wrong and some of his names not valid, but the 

 vast amount of knowledge presented through this paper will be of ever- 

 lasting value to students. 



Cox (1937) has written a very interesting paper on the Jurassic 

 mytilids with general considerations which should be read by students 

 working on the recent material. He describes Falcbiiytilus as a subgenus 

 of Alytilus. This group consists of species with terminal umbones but 

 without lunular grooves or teeth. Even if they should be the ancestors 

 of Mytilus s. L, I believe Falcimytilus should be given generic rank. 



Newell (1942) has treated the late Palaeozoic Mytilacea mainly on 

 American material. His extensive discussion of the shell characters and 

 the origin and evolution of Mytilacea is very useful and interesting. He 

 describes the genus Promytilus and the genus Tolsellina for mytiliform 

 and modioliform late Palaeozoic mytilids respectively. Unfortunately the 

 hinge and the muscle scars are not described. 



The anatomy of several mytilids has been extensively treated by List 

 (1902), Pelseneer (1911), and White (1937), and notes about the 

 anatomy of different species can also be found scattered in the literature. 



What are the characters which can be used in the classification of the 

 variable mytilid species? As the major part of the mytilids have anteriorly 

 placed umbones and are rather alike in form, this resemblance is not 

 necessarily a criterion for the closer relationship of similar appearing 

 species. The same form could easily have been acquired by different 

 evolutionary lines, but it must be possible to prove or disprove this by 

 a closer study of other characters and especially by following these char- 

 acters through the fossil forms. Unfortunately very few characters are 

 preserved in fossils and our knowledge of most of the recent species is 

 still poor. It is, therefore, extremely difficult to tell which characters are 

 more important than others and how the various species groups can be 

 placed in relationship to each other. In this paper, therefore, many small 

 groups are treated as genera; they probably will find their proper places 

 in the system in the future. 



The characters used for classification should, as far as possible, be taken 

 from the shell and from easily visible parts of the anatomy, such as the 

 mantle margin, foot, byssus, etc. One character which is supposed to be 



