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Vol. 36(2), 2001

The jaws consisting of two plates are found in all examined species of cerithioidcans, and are like those seen in several outgroups of vetigastropods (e.g., Haliotis spp.; Simone, 1997). The size of the jaws, however, presents valuable comparative data. Although apparently subjective, careful examination leaves little doubt as to in which category the jaws belong. Most cerithioideans have jaws of medium size, but in Campanile the jaws are exceptionally large (fig. 416) (Houbrick, 198la). In Supplanaxis and Cerithidea the jaws are very smali, almost vestigial, as two smali triangular plates attached to a flaccid transparent membranę (fig. 74). The functional utility of each jaw type is an interesting topie for futurę research.

Odontophore

The cerithioidean odontophore is different of any other known among the prosobranchs, both archaeogastropods and higher caenogastropods, in lacking muscles in both extremities of the radular ribbon. This character could be extremely valuable if the odontophore of littorinids and hydrobioids were not surprisingly similar. Although the names for each muscle are given in the dcscriptions, obviously these can be changed with further functional or phylogenetic analysis. For this reason, each muscle (or pair of muscles) is referred to by its number (e.g., m4). Although functional experiments have not been performed, muscle function can be suggested based on the anatomical disposition. The radula in the cerithioidean odontophore does not appear to work in a back-and-forth movement of the radular ribbon, gliding on cartilages, as in most other gastropods. The muscles of the cerithioidean odontophore apparently work firmly holding the radula and the subradular cartilage to the odontophore, and the whole odontophore works as a grazer. Apparently the glide between radula and cartilages is absent.

The configuration of the musculature of the cerithioideans odontophore provides valuable systematic and phylogenetic data, but has been poorly examined. Some data is given by Starmuhler (1969, on Melanatria, Cleopatra and Melanoides) which, together with the data given herein, may be the basis of further systematic and functional studies.

60.    M2: 0= present; 1= absent (Turritella hookeri,

Campanile symbolicum, Serpulorbis decussatus)-, 2= narrow and long, inserting posteriorly (cerithids, Finella dubia, Alaba incertd) (CI— 66, RI= 83, not additive).

The pair of retractor of buccal mass (m2) and the horizontal muscle (m6) are the only muscles with morę obvious correlation with odontophore muscles of the other gastropods. The pair m2 is, however, absent in Campanile, Turritella and Serpulorbis, the loose condition is regarded as apomorphic.

61.    M3: 0= absent; 1= present (Aylacostoma

exoplicata, A. ci) (CI= 100, RI= 100).

62.    M4: 0= tumed posterior; 1= tumed anterior

(Serpulorbis decussatus) (CI= 50, Ri= 0). The m4 (and other muscles) are inverted in 5. decussatus when compared with those of other cerithioideans, a condition shared with Littorina flava (Simone, 1998), and probably is an outgroup convergence.

63.    M5 origin in m4: 0= median; 1= lateral (all

cerithioideans) (CI= 100, Rl= 100).

64.    Union m5-m5: 0= absent; 1= present (all

cerithioideans except hi. modulus) (CI^ 100, RI= 100).

65.    Horizontal muscle (m6): 0= normal (somewhat

thin); 1= very thick (Campanile symbolicum) (CI= 100, RI= 100).

66.    Site of m6: 0= dorsal; 1= anterior-ventral

(Serpulorbis decussatus) (CI= 50, RI= 0). Other character shared with Littorina flava and also probably due to inversion of odontophore.

67.    Anterior thickness of m6:0= absent; 1 = present

(pleurocerids) (CI= 100, RI= 100).

68.    Connection of m7 with m4: 0= present; 1 =

absent (thiarids, Doryssa spp., cerithids, Finella dubia, Alaba incerta, Campanile symbolicum) (CI= 25, RI= 70).

69.    Connection m7-ml 1 with snout: 0= absent; 1=

present and anterior (Aylacostoma exoplicata,